title: How can I remove a default gem? ! want to uninstall a gem 1.7.7 version of json
body: <p>I have the same rails app in OSX and Ubuntu, I want to use Zeus to speed up my rspec.
  In Ubuntu, Zeus starts Ok, but in OSX it always be crashed. 
  At last I find the issue, <a href="https://github.com/burke/zeus/issues/237#issuecomment-18700462" rel="noreferrer">https://github.com/burke/zeus/issues/237#issuecomment-18700462</a> the difference between OSX and Ubuntu is the version of json gem.
  I use <code>gem list | grep json</code>
  Ubuntu shows</p>
  
  <pre><code>json (1.8.1, 1.8.0, 1.5.3)
  json_pure (1.5.3)
  json_spec (1.1.1)
  jsonpath (0.5.3)
  multi_json (1.8.2, 1.7.8, 1.0.3)
  </code></pre>
  
  <p>Mac shows </p>
  
  <pre><code>json (1.8.1, 1.7.7)
  json_spec (1.1.1)
  jsonpath (0.5.5, 0.5.3)
  multi_json (1.8.2, 1.7.8)
  </code></pre>
  
  <p>so I want to uninstall 1.7.7 version of json gem to make zeus start, but </p>
  
  <pre><code>gem uninstall json -v 1.7.7
  ERROR:  While executing gem ... (Gem::InstallError)
  gem "json" cannot be uninstalled because it is a default gem
  </code></pre>
  
  <p>What should I do?</p>