Rbarlow celery 3.1.11

This pull request upgrades us to these package versions:

python-amqp-1.4.5
python-billiard-3.3.0.17
python-kombu-3.0.15 (with @bmbouter's patch)
python-celery-3.1.11 (now without a prior patch, which has been accepted upstream in this version)

I have not yet done any tito tagging, as it was expressed that we should do PR review first.