[bug 785856] Upgrade celery to v3.0.24.

* Updates celery to v3.0.24
* Updates django-celery to v3.0.23 (there is no v3.0.24 for django-celery)
* Adds amqp v1.4.2 replacing amqplib v1.0.2
* Small code changes to match the new API for celery
* Migration that adds missing tables for celery