title: Caching pipenv / Pipfile dependencies on TravisCI
body: <p>The Travis <a href="https://docs.travis-ci.com/user/caching/" rel="noreferrer">documentation</a> on caching does not specifically mention how to cache python dependencies installed from pipenv's Pipfile, rather than from pip's usual <code>requirements.txt</code>. I tried setting up pip caching per documentation anyway, but build times are not improved at all, and I see pipenv installing its deps on every run. </p>
  
  <p>This is the syntax I'm currently using - what is the correct syntax? (or is it even supported?)</p>
  
  <pre><code>language: python
  python:
    - "3.6"
  
  cache: pip
  
  cache:
      directories:
          - proj/static/node_modules
          - $HOME/.cache/pip
  </code></pre>