This change makes ping into a message type, which
makes the whole protocol a lot more consistent.

Also fixes hacks on the client side to make this all
work.

* Move specific requires close to where they're needed.
* Use the private active_support/rails dep to wrap up common needs
  like eager autoload and module delegation.
* Use a single Rails engine rather than an engine and a railtie.
* Prefer `AS::JSON.encode` to `Object#to_json`.

Remove out-of-data autoload reference of ActionCable::Broadcaster that removed at e1a99a83 .

The issue: https://github.com/celluloid/celluloid/issues/637

You’d never instantiate it on its own.