Fix for extracting version number in Python 3

setuptools chokes when installing a tarball of django-compressor's
HEAD of develop under Python 3. The flaw lies with extracting the
version number.

As suggested by @jezdez, this commit merely ports the version
extracting code from django-configurations.
Bump version number to 1.4a1

As 1.3 has been released, the version number should be bumped.
Merge pull request #463 from maikhoepfel/fix/setuppy

Fix for extracting version number in Python 3