add redis to travis builds

.travis.yml

@@ -1,6 +1,7 @@
 sudo: required
 services:
   - postgresql
+  - redis-server
 addons:
   postgresql: "9.4"
 language: python

netbox/utilities/api.py

@@ -250,7 +250,6 @@ class ModelViewSet(_ModelViewSet):
         # Fall back to the hard-coded serializer class
         return self.serializer_class
 
-
     @method_decorator(cache_page(settings.CACHE_TIMEOUT))
     def list(self, *args, **kwargs):
         """