Dead links in boilerplate markup

[Lighton Phiri]

• In the 'Getting set up' subsection on page 33, the links to underscore.js [1] and backbone.js [2] are dead.
• I believe this is the case for the jsFiddle and jsBin versions as well.

[1] http://documentcloud.github.com/underscore/underscore-min.js
[2] http://documentcloud.github.com/backbone/backbone-min.js