Install ruby headers with rvm
===

Travis CI uses RVM to provide Ruby, but it doen't seem to contain the headers:

    $ find /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/zlib.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/dl.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/syck.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/socket.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/dbm.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/pty.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/strscan.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/io
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/io/console.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/io/wait.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/io/nonblock.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/iconv.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/iso_8859_14.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/gb2312.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/iso_8859_16.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/windows_1251.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/euc_jp.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/iso_8859_4.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/iso_8859_1.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/japanese_euc.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/emoji_sjis_softbank.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/single_byte.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/emoji_sjis_docomo.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/utf8_mac.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/gbk.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/emoji.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/japanese_sjis.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/big5.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/transdb.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/japanese.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/gb18030.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/utf_16_32.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/korean.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/escape.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/emoji_iso2022_kddi.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/chinese.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/iso2022.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/trans/emoji_sjis_kddi.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/encdb.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/utf_16le.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/gbk.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/iso_8859_8.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/euc_tw.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/koi8_r.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/iso_8859_9.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/utf_32le.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/big5.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/shift_jis.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/cp949.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/gb18030.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/koi8_u.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/iso_8859_7.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/utf_32be.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/euc_kr.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/iso_8859_15.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/iso_8859_10.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/iso_8859_3.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/utf_16be.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/emacs_mule.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/iso_8859_2.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/iso_8859_6.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/iso_8859_13.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/iso_8859_5.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/enc/iso_8859_11.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/json
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/json/ext
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/json/ext/generator.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/json/ext/parser.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/curses.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/fiber.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/racc
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/racc/cparse.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/stringio.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/bigdecimal.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/syslog.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/psych.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/rbconfig.rb
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/gdbm.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/date_core.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/pathname.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/etc.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/digest.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/dl
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/dl/callback.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/openssl.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/digest
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/digest/md5.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/digest/rmd160.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/digest/sha2.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/digest/sha1.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/digest/bubblebabble.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/sdbm.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/mathn
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/mathn/rational.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/mathn/complex.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/continuation.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/fcntl.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/objspace.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/readline.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/ripper.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/nkf.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/fiddle.so
    /home/vagrant/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/i686-linux/coverage.so


How can I install the missing headers (`ruby.h` and friends)?