adding hidden {{lastUpdated}} to template so that observers are correctly setup

This is a quick fix, changing step models was not resulting in the [lastUpdated](https://github.com/GavinJoyce/ember-beats/blob/master/app/models/mixins/channel-serialization.js#L4) updating correctly.

I need to research why this hack is needed, I though that getting the `lastUpdated` property `.on('init')` would be enough:

https://github.com/GavinJoyce/ember-beats/blob/master/app/models/channel.js#L15