celery: upgrade to Celery 3

Celery 3 is more than 2 and is the future and should be more stable than Celery 2.

There might be problems like
PRECONDITION_FAILED - cannot redeclare exchange 'celeryresults' in vhost 'kallithea' with different type, durable, internal or autodelete value
when celery is upgraded and it tries to upgrade existing tasks.

One way to avoid that problem:
- make sure celery has run to completion and there are no important pending tasks,
- delete the old vhost,
- upgrade celery,
- create new vhost
- set permissions for vhost
- profit