`DjangoVumiApiHelper` is a wrapper around `VumiApiHelper` and therefore needs to have both `is_sync` and `use_riak` properties. However, since it's only useful in Django-based tests, `is_sync` will *always* be `True` on `DjangoVumiApiHelper`. I can move that to the constructor instead of making it a class attribute if you'd prefer, but it doesn't make sense to have a constructor parameter for a thing that can only ever have one value.