title: Git-backed pip dependencies on Heroku are not upgraded
body: <p>I'm deploying a Python application to Heroku.</p>
  
  <p>I have a <code>requirements.txt</code> file in which I install a dependency from a git repo, pinned to a certain tag, let's say:</p>
  
  <pre><code>git+git://github.com/django/django.git@1.7c2#egg=django
  </code></pre>
  
  <p>When I redeploy this, e.g. after changing the tag reference to <code>1.7c3</code>, Heroku does not update this dependency. (As can be seen with <code>heroku run pip list</code>.)</p>
  
  <p>As a workaround, I have found that you can modify your <code>runtime.txt</code> (make it reference an older Python version), commit, push to Heroku (which rebuilds the entire environment), then undo the commit and force push to Heroku again (which rebuilds the entire environment again). This is of course not a very satisfying solution in the long-term (and I don't like the idea of having to deploy my production app twice).</p>
  
  <p>Is there a better solution? Any improvements on the horizon?</p>