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.

Is this the solution?

It works, but I'm not sure if shimming is better. 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.