make sure that wheel files with a single digit version do work

this is a rather serious bug, since it makes 'pip wheel ...' fail with
the following error if the wheelhouse contains such a wheel:

,----
| ...
|   File "/home/ralf/.ve/dev/local/lib/python2.7/site-packages/pip/wheel.py", line 420, in __init__
|     self.version = wheel_info.group('ver').replace('_', '-')
| AttributeError: 'NoneType' object has no attribute 'replace'
`----