Updated to Celery 3.1.5.

We have been pinned at an old version of celery (3.0.22) due to a bug that was later fixed. The djcelery app for integrating with Django and `celeryd` are both deprecated. Celery integrates directly with Django now in essentially the same way it would for any generic project.

This removes a django app and several unused database tables (djcelery), moves the celery CONCURRENCY setting to settings.py (or local_settings.py). There is one post-3.1.5 celery bugfix we need, so we must pull from a git SHA until they package a newer celery.