Changing SESSION_SERIALIZER to PickleSerializer, quickfix for new Django version

django-carrots uses an old version of django-social-auth which doesn't support changes in Django since 1.5.3 (1.6).
https://github.com/omab/python-social-auth/issues/36
Merge pull request #4 from pbetkier/patch-2

Changing SESSION_SERIALIZER to PickleSerializer, quickfix for new Django...