Fix authentication check for django 2.0 (jazzband#445)

The method 'user.is_authenticated()' is deprecated,
and it is removed in the v2 release of django.

push_notifications/api/rest_framework.py

@@ -151,12 +151,12 @@ def create(self, request, *args, **kwargs):
 			return Response(serializer.data, status=status.HTTP_201_CREATED, headers=headers)
 
 	def perform_create(self, serializer):
-		if self.request.user.is_authenticated():
+		if self.request.user.is_authenticated:
 			serializer.save(user=self.request.user)
 		return super(DeviceViewSetMixin, self).perform_create(serializer)
 
 	def perform_update(self, serializer):
-		if self.request.user.is_authenticated():
+		if self.request.user.is_authenticated:
 			serializer.save(user=self.request.user)
 		return super(DeviceViewSetMixin, self).perform_update(serializer)