Fix a Django deprecation warning
Fix a Django deprecation warning

Replace deprecated assignment_tag:

`python/lib/python3.5/site-packages/admin_interface/templatetags/admin_interface_tags.py:11: RemovedInDjango20Warning: assignment_tag() is deprecated. Use simple_tag() instead`

fabiocaccamo/django-admin-interface,fabiocaccamo/django-admin-interface,fabiocaccamo/django-admin-interface