Bump to Django 1.6
Fix bug in serializers dealing with related topic fields

In Django 1.6, GenericRelation fields are no longer included in
model._meta.many_to_many, which broke the automatic detection of related
topic fields in our serializers. Instead of the automatic detection,
just explicitly map related topic fields to the TopicAssignmentField.

see https://code.djangoproject.com/ticket/21428 regarding the
GenericRelation change
Bump reversion to 1.8.4

Reversion 1.8.* stopped creating revisions upon deletion, so we remove
those. Migration 0137 is a duplicate of reversion's 0006 migration, but
South doesn't respect cascading delete in non-frozen apps, apparently.
Tryingto run that one (reversion's 0006) would violate the FK
requirement in postgres for our RevisionProject model, since django
deals with ON DELETE behavior instead of baking it into the database. I
know, ugh.