title: Failed to install Rails on yosemite
body: <p>I installed Ruby via <code>rbenv</code>. Currently, my pc has ruby <code>2.1.2p95</code> version installed. I have just started development in rails. So, don't know much about it. 
  But, I get some kind of error when I installed it. </p>
  
  <pre><code> $gem install rails -v 4.2.0
   ERROR:  Loading command: install (LoadError)
           cannot load such file -- zlib
   ERROR:  While executing gem ... (NoMethodError)
           undefined method `invoke_with_build_args' for nil:NilClass
  </code></pre>