Fix djcelery dependency for Celery 3.1

`Celery 3.1` doesn't depend on `djcelery` to work with Django anymore.

This pull request is aiming to fix bug described in https://github.com/getsentry/raven-python/issues/427.
I've tested it with `celery==3.1.10` and `sentry==6.4.4`.
This will not be acceptable as it induces that celery should be installed along with Django which is usually not required.