Update libs.rst

updated django-celery url to point to celery/django-celery (which looks like it's being maintained) instead of the fork ask/django-celery (which hasn't been updated since 2012). It looks like funfactory uses this version as well (https://github.com/mozilla/funfactory/blob/master/funfactory/requirements/prod.txt).
Merge pull request #31 from bjdixon/patch-2

Update libs.rst