Use triggerMethod on child view if available

This PR fixes #1488.  It abstracts out the logic a collection view uses for calling `triggerMethod` on a child view.  If the child view implements `triggerMethod`, then it calls that.  Otherwise, it calls its own implementation on the child view.

I cleaned up one of the specs that was kind of related, but we probably want specs that actually verify that the correct `triggerMethod` is called.

Please look over this to make sure this satisfies the requirements of #1488 and isn't too crazy :sleeping: 
I think `triggerMethodOn` would be a more generic solution. We have the same kind of case statement in regions as well.

https://github.com/marionettejs/backbone.marionette/issues/1558