title: Downgrading package with pip produces ValueError('Expected Version spec in',
body: <p>On an Ubuntu 12.04 system where I don't have sudo rights, I've setup a virtual environment so I can have up to date python packages. The system virtualenv installed a pretty old version of pip into it (1.1) which I upgraded to 6.1.1.</p>
  
  <p>I find now that if I try to specify a package version with something like:</p>
  
  <pre><code>pip install pymongo=2.6.2
  </code></pre>
  
  <p>pip throws a `ValueError('Expected version spec in', 'pymongo=2.6.2', 'at', '=2.6.2')</p>