Backbone and Underscore package version updates

[Backbone.js 1.0](http://backbonejs.org/#changelog) is a big milestone. We've been using it in production at artsy.net for several weeks.
This update requires Underscore.js >= 1.4.3. This commit updates the `underscore` package to [the latest version, 1.4.4](http://underscorejs.org/#changelog).