Add support for Django 1.7+'s caches

Review Request #8721 — Created Feb. 6, 2017 and discarded — Latest diff uploaded

Information

Djblets
release-0.10.x

Reviewers

In Django 1.7, there's a new caches object which supplants the old
get_cache method. This change adds support for that.

Ran Review Board unit tests with log capture disabled. Saw that the
RemovedInDjango19 warnings disappeared.