Update requirejs shim text regarding underscore/backbone

In [chapter 8](https://github.com/addyosmani/backbone-fundamentals/blob/gh-pages/chapters/08-modular-development.md):

> Ideally, each library that we use with RequireJS will come with AMD support. That is, it uses the `define` method to define the library as a module. However, some libraries - including Backbone and one of its dependencies, Underscore - don't do this.

It looks like this is no longer true.
[That section](https://github.com/addyosmani/backbone-fundamentals/commit/71d004046b93982397f6df694d5ea8f4803818d0) was written in February 2013.
Backbone appears to have [added AMD support](https://github.com/jashkenas/backbone/commit/ea29bd9119acffaf2a24ff0a362304210fc2c019) in November.
Likewise [for Underscore](https://github.com/jashkenas/underscore/commit/d12b5beb6156bde79d1532b429c925bae8025ff8).

So the shims may not be necessary anymore, but obviously they are still a good thing to explain. Maybe the text should simply be updated to say something like "while Backbone and Underscore have added AMD support, we're going to show an example of how to shim a library that does not."

Do you agree?