Make Marionette work with Browserify

This PR fixes Issue #2074.

Marionette now works with Browserify as Wreqr and BabySitter are attached to Backbone. 
@commenthol, you're right that Wreqr isn't used here, but the solution isn't to attach it to Marionette. @thejameskyle, we don't lint this file.

We need to require Wreqr so that it's attached to Backbone. Same with Babysitter. We don't need to keep a reference to it, though, so it could just be:

```js
require('backbone.wreqr');
require('backbone.babysitter');
```