Replaces slice with _.toArray

This is required before #1900 can land. The fact that we have a weird function just hanging around 'in-scope' makes it impossible to use Marionette per-component, unless you specify that helper within each UMD wrapper. And that'd be pretty yucky.

While the motivation for this was originally for the sake of getting that PR in, it's also just better this way, I think. `toArray` is self-describing, whereas `slice.call` requires a bit more knowledge of Javascript to understand.

Work on #1900 is kind of halted until this gets in. If you had a quick moment to :eyes: it'd be much appreciated!