Celery upgrade

[davidread]

The Travis build fails because it is on a newer python version. The solution is to upgrade Celery from 2.4 to 3.1 (latest).

The Travis error is:

  File "/home/travis/virtualenv/python2.7.12/lib/python2.7/site-packages/kombu/utils/__init__.py", line 17, in <module>
    from uuid import UUID, uuid4 as _uuid4, _uuid_generate_random
ImportError: cannot import name _uuid_generate_random

Which I understand is this kombu issue, only associated with Python 2.7.11. Kombu fixes it in 3.0.30 (with no backports).

We are on Celery 2.4.2 and Kombu 2.1.3. This the latest Kombu that Celery 2.4 supports.

Celery 3.1 supports Kombu<3.1

We currently run Python 2.7.3 (precise 12.04). Trusty 14.04 uses Python 2.7.5. Xenial 16.04 will need this patch too - Python 2.7.11.