title: bundle install using wrong ruby with rvm when building native extension?
body: <p>I'm using <code>bundle install</code> to install <code>rest-client</code>. It's failing with the following:</p>
  
  <pre><code>$ bundle
  Fetching gem metadata from https://rubygems.org/..........
  Fetching version metadata from https://rubygems.org/..
  Resolving dependencies...
  Installing unf_ext 0.0.7.4 with native extensions
  
  Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
  
      current directory: /home/roger/.rvm/gems/ruby-2.3.4@junk-drawer/gems/unf_ext-0.0.7.4/ext/unf_ext
  /usr/bin/ruby2.3 -r ./siteconf20180115-9722-1iow75n.rb extconf.rb
  mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h
  
  extconf failed, exit code 1
  
  Gem files will remain installed in /home/roger/.rvm/gems/ruby-2.3.4@junk-drawer/gems/unf_ext-0.0.7.4 for inspection.
  Results logged to /home/roger/.rvm/gems/ruby-2.3.4@junk-drawer/extensions/x86_64-linux/2.3.0/unf_ext-0.0.7.4/gem_make.out
  Installing mime-types-data 3.2016.0521
  Installing netrc 0.11.0
  Using bundler 1.11.2
  An error occurred while installing unf_ext (0.0.7.4), and Bundler cannot continue.
  Make sure that `gem install unf_ext -v '0.0.7.4'` succeeds before bundling.
  </code></pre>
  
  <p>I'm using rvm, which appears to be working correctly, if I'm reading the output from <code>rvm info</code> right.</p>
  
  <p><code>which ruby</code> reports <code>/home/roger/.rvm/rubies/ruby-2.3.4/bin/ruby</code>, which looks right.</p>
  
  <p>I found <a href="https://stackoverflow.com/q/11575231/8446">Install ruby headers with rvm</a>, but I <em>have</em> Ruby headers in (apparently) the correct place:</p>
  
  <pre><code>$ ruby -rmkmf -e 'print RbConfig::CONFIG["rubyhdrdir"]'
  /home/roger/.rvm/rubies/ruby-2.3.4/include/ruby-2.3.0
  $ ls /home/roger/.rvm/rubies/ruby-2.3.4/include/ruby-2.3.0
  ruby/  ruby.h  x86_64-linux/
  </code></pre>
  
  <p>In the output from <code>bundle</code>, I see <code>/usr/bin/ruby2.3</code>, which looks suspicious.</p>
  
  <p><strong>It's looking in the wrong place for Ruby header files. How do I get it to use the RVM-installed ones?</strong></p>