Define a __hash__ on the packaging.version.Version subclasses

In Python 3.x a subclass will not inherent the __hash__ method from
the parent classes if the subclass defines a __eq__ method. This
means that without defining our own __hash__ the SetuptoolsVersion
classes are unhashable.
Merge pull request #18 from dstufft/hashable-versions

Define a __hash__ on the packaging.version.Version subclasses