title: NoMethodError: undefined method `has_attached_file'
body: <p>Paperclip produces this error, after checking out the plugin's rails3 branch. 
  My Gemfile has following line: </p>
  
  <pre><code>gem 'paperclip', :git =&gt; 'http://github.com/thoughtbot/paperclip.git', :branch =&gt; 'rails3'
  </code></pre>
  
  <p>And the error message is: </p>
  
  <pre><code>NoMethodError: undefined method `has_attached_file' for #&lt;Class:0x2a50530&gt;
  </code></pre>