add django-celery to the requirements

requirements/prod.txt

@@ -9,7 +9,7 @@ South==0.7
 suds==0.3.9
 pytz==2010e
 GitPython==0.1.7
-celery==1.0.2
+django-celery==2.0.2
 # This is required by celery, but celery's setup.py does bad things and
 # packaging doesn't pick it up.
 importlib==1.0.2