title: Can't install json gem 1.8.6 - failed to build gem native extension on Ubuntu
body: <p>Bundler on Ubuntu 16.04 yields the following error when trying to install JSON. </p>
  
  <pre><code>Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
  
      current directory: /tmp/bundler20170601-12986-10dsu07json-1.8.6/gems/json-1.8.6/ext/json/ext/generator
  /usr/bin/ruby2.3 -r ./siteconf20170601-12986-op73sp.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 /tmp/bundler20170601-12986-10dsu07json-1.8.6/gems/json-1.8.6 for inspection.
  Results logged to /tmp/bundler20170601-12986-10dsu07json-1.8.6/extensions/x86_64-linux/2.3.0/json-1.8.6/gem_make.out
  
  An error occurred while installing json (1.8.6), and Bundler cannot continue.
  Make sure that `gem install json -v '1.8.6'` succeeds before bundling.
  
  In Gemfile:
    react-rails was resolved to 1.7.2, which depends on
      rails was resolved to 4.2.7.1, which depends on
        actionmailer was resolved to 4.2.7.1, which depends on
          actionpack was resolved to 4.2.7.1, which depends on
            actionview was resolved to 4.2.7.1, which depends on
              rails-dom-testing was resolved to 1.0.8, which depends on
                rails-deprecated_sanitizer was resolved to 1.0.3, which depends on
                  activesupport was resolved to 4.2.7.1, which depends on
                    json
  </code></pre>
  
  <p>It looks as if the gem extensions couldn't be compiled because of lacking header files. What can I do?</p>