Change the Marionette.extend reference to be Backbone.View.extend

Since Marionette is essentially a bunch of extensions to Backbone.View, it makes sense that `Marionette.extend` would be a reference to `Backbone.View.extend` (and not `Backbone.Model.extend`).

Note that `Backbone.Model.extend` and `Backbone.View.extend` are simply references to the same internal `extend` helper function in Backbone, so this PR has absolutely no effects. But it looks a bit strange to see `Backbone.Model` in Marionette's codebase. This change brings a little more coherence.