Fix download URL in setup.py

https://github.com/downloads/deschler/django-modeltranslation/django-modeltranslation-0.5b1.tar.gz returns 404.
Whereas https://github.com/deschler/django-modeltranslation/archive/0.5b1.tar.gz is ok.

This pull-request fixes the download URL.

Note: the download URL seems used by either zc.recipe.egg or z3c.recipe.scripts, i.e. broken download URL means download problems with buildout.