Removed call to with_metaclass as it is no longer needed
Removed call to with_metaclass as it is no longer needed

Subfieldbase is deprecated and scheduled to be removed with Django 1.10

Fixes #257

nnseva/django-push-notifications,shigmas/django-push-notifications,cristiano2lopes/django-push-notifications,Tictrac/django-push-notifications,CustomerSupport/django-push-notifications,fsto/django-push-notifications,GaleDragon/django-push-notifications,jleclanche/django-push-notifications,gkirkpatrick/django-push-notifications,lukeburden/django-push-notifications,Adys/django-push-notifications,jamaalscarlett/django-push-notifications,hylje/django-push-notifications,rsalmaso/django-push-notifications,1vank1n/django-push-notifications,matthewh/django-push-notifications