Seems a bit odd to set `is_sync=True` here and on the class? Maybe `is_sync` should be an argument too like it is for `VumiApiHelper`? I think `use_riak` should go onto `VumiApiHelper` the same way `is_sync` does -- it seems generally useful to be able to tell what sort of vumi_helper one has. Maybe `DjangoVumiApiHelper` could have properties that proxy to `vumi_helper` for these two attributes if needed (although they don't appear to be used in the class itself).