fix deprecation warning for newer celery

@andymckay r?

Without this you get this deprecation warning every time the file is loaded:

```
/Users/peterbe/dev/MOZILLA/ELMO/elmo/vendor/lib/python/celery/decorators.py:33: CDeprecationWarning: 
The `celery.decorators` module along with the magic keyword arguments,
are deprecated, and will be removed in version 3.0.

Please use the `celery.task` module instead of `celery.decorators`,
and the `task.request` should be used instead of the magic keyword arguments:

    from celery.task import task

See http://bit.ly/celery22major for more information.
```