Update Celery to version 3.1.7 and Django-celery to version 3.1.1

The integration of Celery in Django has changed from version 3.1 on,
django-celery is not required anymore to start tasks, etc.. However, it might
still be of use for CATMAID, since it can be used for the database result
backend and the Django periodic task admin.
Remove django-kombu, because of it's inclusion in kombu [settings update]

Please update your settings files according to the modified settings.py.example:
Replace the lines

  INSTALLED_APPS += ("djkombu",)
  BROKER_BACKEND = "djkombu.transport.DatabaseTransport"

by these lines:

  INSTALLED_APPS += ("kombu.transport.django",)
  BROKER_URL = 'django://'