Be consistent with _.each iteration

`_.each` should be more rigid going forward (jashkenas/underscore#1990) (both underscore and lodash will be using similar check in their respective next versions).

Along with https://github.com/megawac/backbone.marionette/commit/504eca1710b59394c3fe6b921bbc80fdf814b5f6 this fixes #2108. Thats all there is to it