Uses listenTo instead of on.

This is really just a change to follow the best practices of using `on` vs. `listenTo`. As far as I know, there's no functional change here, which is a consequence of neither models nor collections calling `off` or `stopListening` in any other circumstance.
Passing `model, 'all', this._onModelEvent)` would more precisely undo the set-up of this listener, but just passing `model` seems to have more fidelity to the comment.