Fixed issue with the lost of events

Steps to reproduce:

    view1 = /* ... */;
    view2 = Backbone.Marionette.Layout.extend({ /* ... */, events : {
      'evt element' : 'handler'
    }});
    region.show(view2); // here we can handle `handler`
    region.show(view1);
    region.show(view2); // but here not