Fix celery dependency for review bot worker.
Review Request #8759 — Created Feb. 17, 2017 and discarded
The Review Bot extension depends on Celery < 4.0 (since that has a Django 1.8+
dependency itself), but I had neglected to update the worker to do the same.
Trivial change.
Tool: PEP8 Style Checker Processed Files: bot/setup.py Tool: Pyflakes Processed Files: bot/setup.py