Support Django 1.4, 1.5 and 1.6

The default session serializer switched to JSONSerializer in Django-1.6.
Unfortunately, it can't serialize arbitrary objects (such as datetime
instances), there we have to stay with PickleSerializer (compare with
https://docs.djangoproject.com/en/1.6/topics/http/sessions/#write-your-own-serializer).

Sets Django==1.4 for the py27dj14 tox testenv. Don't relax the version
requirements just yet.

Change-Id: Ifb5a68950fa6a4a652a0fb0cb81048d89763ec3f
Merge "Support Django 1.4, 1.5 and 1.6"