Trigger a `remove` on last superset's model

When the last item of a collection is removed, the `remove` event isn't fired by the paginated collection.

It's due to the fact that `updateNumPages` return true when the number of pages is 0, even if the superset is empty, and so `onAddRemove` return early instead of removing that last model from it's wrapped collection.

See this [reduced test-case on jsBin](http://jsbin.com/xuxavotowa/edit?html,js,output)