Relax django-celery constraint so package works Celery 2.5
Relax django-celery constraint so package works Celery 2.5

Fixes #4

michaelkuty/django-cacheback,bpeschier/django-cacheback,michaelkuty/django-cacheback,codeinthehole/django-cacheback,bharling/django-cacheback,bpeschier/django-cacheback,codeinthehole/django-cacheback,jayfk/django-cacheback,michaelkuty/django-cacheback,safarijv/django-cacheback,jayfk/django-cacheback,mitchelljkotler/django-cacheback,safarijv/django-cacheback,mitchelljkotler/django-cacheback,bharling/django-cacheback,jayfk/django-cacheback,codeinthehole/django-cacheback,bharling/django-cacheback,safarijv/django-cacheback,mitchelljkotler/django-cacheback,bpeschier/django-cacheback