title: error in pipenv setup command: 'install_requires' must be a string or list of strings containing valid project/version requirement specifiers
body: <p>When I try to pipinstall pipenv I get the following error:</p>
  
  <pre><code>nathan@DESKTOP-KKB4QL2:/mnt/c/Users/Nathan/Dropbox$ pip install pipenv
  Downloading/unpacking pipenv
    Downloading pipenv-11.7.1.tar.gz (5.0MB): 5.0MB downloaded
    Running setup.py (path:/tmp/pip_build_nathan/pipenv/setup.py) egg_info for package pipenv
      error in pipenv setup command: 'install_requires' must be a string or list of strings containing valid project/version requirement specifiers
      Complete output from command python setup.py egg_info:
      error in pipenv setup command: 'install_requires' must be a string or list of strings containing valid project/version requirement specifiers
  
  ----------------------------------------
  Cleaning up...
  Command python setup.py egg_info failed with error code 1 in /tmp/pip_build_nathan/pipenv
  Storing debug log for failure in /home/nathan/.pip/pip.log
  </code></pre>
  
  <p>I've tried to update pip, but that just resulted in an exception. </p>