title: Error with Rails Edge/4 `require': cannot load such file -- thread_safe
body: <p>Following ths instructions here for getting an app running off Rails Edge to test Rails 4:
  <a href="https://stackoverflow.com/questions/346720/what-is-the-best-way-to-generate-a-rails-app-using-edge">What is the best way to generate a Rails app using edge?</a>
  and
  <a href="https://stackoverflow.com/questions/3821077/how-to-create-edge-rails-application">How to create Edge rails application?</a></p>
  
  <p>But running into this error:</p>
  
  <pre><code>`require': cannot load such file -- thread_safe (LoadError)
  </code></pre>
  
  <p>Using the command:</p>
  
  <pre><code>rails new Edge --edge --skip-bundle --database=postgresql --skip-test-unit --skip-index-html
  </code></pre>
  
  <p>A checkout of Rails master bin dir is on my path.</p>
  
  <p>I am using rbenv with ruby 1.9.3p327.</p>
  
  <p>I have cleared all my installed gems out, just running bundler. </p>
  
  <p>Rails 3.2.9 works fine on my system, have an app in active development. </p>