title: What is the correct way to specify gems that I only want included in certain os environments?
body: <p>Recently I started add this to my Gemfile.</p>
  
  <pre><code>  gem "autotest"
    gem "autotest-growl" # So you get friendly notices
    gem "autotest-fsevent" if `uname`.chomp == "Darwin" # Autotest OSX Specific gems.  So autotests runs better and uses less processor on a Mac
  </code></pre>
  
  <p>Unfortunately the Gemfile.lock file still ended up on the non mac CI and is now failing the build, meaning the if statement became pointless.</p>
  
  <p>What is the correct way to specify gems that I only want included in certain os environments?</p>
  
  <p><strong>Edit:</strong> Platforms looks cool, unfortunately I want to make it so I can specify gems for OSX but not for our linux server. Further suggestions?</p>