title: Gem install command doesn't work
body: <p>I'm now using macbook terminal to study rails. 
  I'm trying to install ruby on rails so I got homebrew and rbenv. 
  And I entered "gem install rails" to get rails but he said</p>
  
  <blockquote>
    <p>"ERROR:  Loading command: install (LoadError)     cannot load such file
    -- openssl ERROR:  While executing gem ... (NoMethodError)
        undefined method `invoke_with_build_args' for nil:NilClass"</p>
  </blockquote>
  
  <p>I have no idea with this error because i usually use windows.. 
  I thought that gem installed uncompletely so i tried to reinstall gem, 
  but i couldn't even find how to reinstall this. 
  plz help me.. </p>