Fall back to ROOT_URLCONF in SubscriptionPaymentMiddleware

Thanks @ncouture

Fixes dj-stripe#562

djstripe/middleware.py

@@ -73,7 +73,7 @@ def is_matching_rule(self, request):
             return True
 
         # Second we check against matches
-        match = resolve(request.path, request.urlconf)
+        match = resolve(request.path, getattr(request, "urlconf", settings.ROOT_URLCONF))
         if "({0})".format(match.app_name) in EXEMPT:
             return True