title: Native extension error installing JSON gem
body: <p>I've set up the Brightbox Ruby 2.1.0 apt package and encountering a native compilation error installing <code>json</code> package. </p>
  
  <pre><code>$ sudo gem install json -v '1.8.1'
  
  Fetching: json-1.8.1.gem (100%)
  Building native extensions.  This could take a while...
  ERROR:  Error installing json:
  ERROR: Failed to build gem native extension.
  
      /usr/bin/ruby2.1 extconf.rb
  mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h
  
  extconf failed, exit code 1
  </code></pre>
  
  <p>I've installed <code>ruby2.0-dev</code> as mentioned <a href="https://github.com/brightbox/deb-ruby2.0/issues/1" rel="nofollow">here</a>, but it puts the headers in a different folder (/usr/include/ruby-2.0.0/ruby, and copying them to /usr/lib/ruby/include didn't help). Any idea how to install this gem?</p>