0.15.1.post1 packaging/versioning problem

Hi, 

There’s a problem with the latest release version — even if you install the `post1` version, it doesn’t register and still shows a need to upgrade. Happens with `pip`/`pip2`/`pip3`.


~$ pip list --outdated 
Package  Version Latest       Type 
-------- ------- ------------ -----
docutils 0.15.1  0.15.1.post1 sdist

~$ pip install --upgrade docutils
Collecting docutils
Installing collected packages: docutils
  Found existing installation: docutils 0.15.1
    Uninstalling docutils-0.15.1:
      Successfully uninstalled docutils-0.15.1
Successfully installed docutils-0.15.1

~$ pip list --outdated 
Package  Version Latest       Type 
-------- ------- ------------ -----
docutils 0.15.1  0.15.1.post1 sdist