title: Failed to Build Gem Extension Error Installing rails
body: <p>I'm on a fresh Ubuntu installation and using the guide to install rails found <a href="http://railsapps.github.io/installrubyonrails-ubuntu.html" rel="nofollow">here</a>.</p>
  
  <p>When I get to the final <code>sudo gem install rails</code>, I get this error:</p>
  
  <pre><code>    Building native extensions.  This could take a while...
  ERROR:  Error installing rails:
      ERROR: Failed to build gem native extension.
  
      current directory: /var/lib/gems/2.3.0/gems/nio4r-1.2.1/ext/nio4r
  /usr/bin/ruby2.3 -r ./siteconf20160821-19665-u7rxlt.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 /var/lib/gems/2.3.0/gems/nio4r-1.2.1 for inspection.
  Results logged to /var/lib/gems/2.3.0/extensions/x86_64-linux/2.3.0/nio4r-1.2.1/gem_make.out
  </code></pre>