title: No module named 'virtualenvwrapper' suddenly while in a virtual environment session
body: <p>I was using <code>virtualenvwrapper</code> just fine when suddenly I started getting </p>
  
  <pre><code>Error while finding spec for 'virtualenvwrapper.hook_loader' (&lt;class 'ImportError'&gt;: No module named 'virtualenvwrapper')
  </code></pre>
  
  <p>and I'm really confused why this is happening? What's changed?</p>