Update broken links to libs

[Jade Misenas]

Links to the libs in chapter 3 here

<script src="http://documentcloud.github.com/underscore/underscore-min.js"></script>
<script src="http://documentcloud.github.com/backbone/backbone-min.js"></script>

don't work anymore but still says you can copy and paste the code below.
Fix looks straightforward but should it maintain old version or newer lib version?

[Addy Osmani]

Thanks for filing a bug @cmisenas! I think where possible it's good for us to lock into a specific version we know works well for the code examples. Would you like to PR updating the links to use Backbone 1.2.3 and Underscore latest and we can check if the demos still work with it?

[Jade Misenas]

Does the demo code live somewhere else or do you have a recommended way to check if demos still work?