Remove django-reversion dependency

[jacobwegner]

As #33 notes, there is a backwards-incompatible change to django-reversion that breaks semantic versioning.

Rather than pinning to a particular version or updating pinax_teams to be compatible, I'm wondering if we instead just drop the django-reversion requirement.

If desired, membership changes could be tracked via signals and a library like pinax_eventlog, but currently reversion might be a bit overkill for the intended use-case of pinax-teams.

[paltman]

Yes! Let's kill it.

[grahamu]

As mentioned in the "django-20" branch merge, I have kept django-reversion in this project for the initial Django v2.0 compatible release so existing users have a path forward.

Let's examine whether a dev can add django-reversion to pinax-teams models within their own project and remove django-reversion if that works.