Swapping-out for the proper endpoint.

* Read "[A new PubSubHubbub endpoint](http://blog.superfeedr.com/push-endpoint/)" from our friends at Superfeedr.
* [DONUT-408](https://tofurkey.urbanairship.com/browse/DONUT-408)

Apparently, our existing endpoint will disappear on 1 March, but the earlier we make this transition the better.