fix is_authenticated for Django 2.0 support

rest_framework_tracking/mixins.py

@@ -123,7 +123,7 @@ def _get_view_method(self, request):
     def _get_user(self, request):
         """Get user."""
         user = request.user
-        if user.is_anonymous():
+        if not user.is_authenticated:
             return None
         return user