Specify stricter version numbers for test requirements

According to the django-reversion wiki the version number of Django and
django-reversion must match:

https://github.com/etianen/django-reversion/wiki/Compatible-Django-Versions

Django 1.3.7, django-reversion 1.5.7
Django 1.4.5, django-reversion 1.6.6
Django 1.5, django-reversion 1.7