Adds support for dashes in wheel version names

Couldn't find any contribution guidelines, so I apologize in advance if this is off the wrong branch, etc.  It's supposed to be for tag 1.4.1

This fix is for when a package has a dash in their version name (ie python-inotify==0.6-test).  When generating a wheel, the file name becomes python_inotify-0.6_test_blah_blah.whl.  However, when that is uploaded to pypi/local pypi proxy, pip sees the version as 0.6_test and ignores it:

```
Ignoring link http://mypypiproxy/+f/2ca1d470df63cef4c00bafed6cd5e110/python_inotify-0.6_test-cp27-none-linux_x86_64.whl#md5=2ca1d470df63cef4c00bafed6cd5e110 (from http:
//mypypiproxy/+simple/python-inotify/), version 0.6_test doesn't match ==0.6-test
```

The fix simply applies the same rule as is used for egg versioning (replace '_' with '-')