title: ActiveAdmin assets precompile error
body: <p>ActiveAdmin is giving me an </p>
  
  <pre><code>Undefined mixin 'global-reset'.
  </code></pre>
  
  <p>error when it try to run </p>
  
  <pre><code>rake assets:precompile
  </code></pre>
  
  <p>ActiveAdmin is 0.3.4. 
  I have ActiveAdmin and an assets group in my Gemfile with sass, coffee-rails and uglifier.</p>