Use mock for tests in celery tests

[ashwoods]

The celery docs advise against using tasks_always_eager (and is currently failing anyways in my current env)

http://docs.celeryproject.org/en/latest/userguide/testing.html

it recommends using mock to test celery tasks in unit tests.

[onekiloparsec]

Have you seen the tests in the django-110 branch? The tests pass whatever the tasks_always_eager value.

I'd be happy to tweak the code I've made to suit your wishes for the project.