Fixes Bug 59846

Just run sudo pip install -e . --upgrade after merging this
According to http://docs.celeryproject.org/en/latest/changelog.html#version-3-1-8,
there were quite a few problems with terminating workers, starting/stopping workers, etc.
since the version we're using, 3.0.  Updating the version fixed the bug on my machine.

However, it seems that workers are still not being closed or recycled
after they finish their task.  We should look into this more.

Change-Id: I88eb6a73262016a472d5377ca4b9c3d76efbf470
Fixes Bug 60286

Change-Id: I886929a2046f5828e1fb7914a6cb5d5124681e09