Prevent Channels from allowing the creation of CableCar identifier

[leastbad]

I'm pretty sure that if someone declared a channel with...

stream_from "CableCar"

... it would blow up https://github.com/stimulusreflex/cable_ready/blob/master/lib/cable_ready/cable_car.rb

I think that it would be best to raise an exception just before https://github.com/stimulusreflex/cable_ready/blob/d809ad348eb649e37a9e547a79c3986494166b05/lib/cable_ready/channels.rb#L17