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.

See https://github.com/etianen/django-reversion/issues/164 for more on
the rationale for changes in reversion.