title: Capybara/selenium: wait for element to become hidden
body: <p>I need to wait for loading bar (div#loading) to disappear (become display: none) in a cucumber step. I'd expect the following to do the trick</p>
  
  <pre><code>find('#loading').should_not be_visible
  </code></pre>
  
  <p>But it doesn't seem to be waiting. Any ideas how to achieve that?</p>