title: Rails install ERROR: Failed to build gem native extension
body: <p>I am trying to install rails on my mac, but every time I type sudo gem install rails into the command line, I get the following error message. I have been stuck for a while, hope you can help.         </p>
  
  <pre><code>    Building native extensions.  This could take a while...
      ERROR:  Error installing rails:
  ERROR: Failed to build gem native extension.
      /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby 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
  
  
      Gem files will remain installed in /Library/Ruby/Gems/2.0.0/gems/atomic-1.1.14 for inspection       
      Results logged to /Library/Ruby/Gems/2.0.0/gems/atomic-1.1.14/ext/gem_make.out
  </code></pre>