title: Ruby not installing json -v 1.8.1 as part of bundle, ERROR: Failed to build gem native extension
body: <p>I've tried installing using as part of the bundle and using sudo, but I get the same error message every time. I'm currently on <code>Ruby 2.0.0 and Rails 4.1.8</code>. I'm not sure what is going on. The exact error message below: </p>
  
  <blockquote>
    <p>Fetching gem metadata from <a href="https://rubygems.org/" rel="nofollow">https://rubygems.org/</a>..........<br>
    Resolving dependencies...<br>
      Using rake 10.4.2 Using i18n 0.6.11</p>
    
    <p>Gem::Ext::BuildError: ERROR: Failed to build gem native extension.</p>
    
    <p>/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby -r
    ./siteconf20141218-80226-brqexc.rb extconf.rb  mkmf.rb can't find
    header files for ruby at
    /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/include/ruby.h</p>
    
    <p>extconf failed, exit code 1</p>
    
    <p>Gem files will remain installed in
    /Library/Ruby/Gems/2.0.0/gems/json-1.8.1 for inspection. Results
    logged to
    /Library/Ruby/Gems/2.0.0/extensions/universal-darwin-14/2.0.0/json-1.8.1/gem_make.out
    An error occurred while installing json (1.8.1), and Bundler cannot
    continue. Make sure that <code>gem install json -v '1.8.1'</code> succeeds before
    bundling.</p>
  </blockquote>
  
  <p>Any advice appreciated.</p>