Moved Backbone and Underscore to peerDeps

Resolves #1294 in [a less gross way](https://github.com/marionettejs/backbone.marionette/pull/1295).

How does this work?

Putting something as a `dependency` pulls in a new version of that library. This creates a clone of it in your built CommonJS file. When you set something as a `peerDependency` it won't nest it, and you won't make clones of it.

This is important when you need the same instance of the library, as you do with `Backbone.history`

Is this the solution?

It works, but I'm not sure if shimming is the real answer. Backbone sets Underscore as a dep so you'll already get a duplicate of that whether you want it or not. Maybe shimming is just the way to go w. CommonJS.

Will do more research.