update kombu version to prevent 100% CPU usage
update kombu version to prevent 100% CPU usage

This pulls in the most recent version of celery/kombu/master - it
prevents situations where the celery MainProcess would consume 100%
CPU while not distributing any tasks to the workers.

Note: If, locally, you already have kombu==4.0.2, you'll have to run
pip install --upgrade https://github.com/celery/kombu/zipball/b2f21289
to get the new version.

alphagov/notifications-api,alphagov/notifications-api