Fix region view destroy bug

In the issue I logged, I had mentioned that was I attempting a PR but ran into some issues (https://github.com/marionettejs/backbone.marionette/issues/1911#issuecomment-56288130). The issue I was running into was actually addressed in another PR that I just noticed (https://github.com/marionettejs/backbone.marionette/pull/1924/files). I am wondering if this line should now be changed to this so we don't unbind all destroy events?
```
this.currentView.off('destroy', this.empty, this);
```