Updating the SimplePush example code to fall in line with recent AG-js c...

...hanges
isn't the "update URL" called `pushEndpoint` on the JSON returned after registration ? 

I am getting [this raw message](https://gist.github.com/matzew/453ffb2db86c280aed01) when trying a registration against MOZ, based on https://gist.github.com/matzew/cbda360d72eaaef75971