Add spring to the application.

Spring (https://github.com/rails/spring) is a Rails app preloader. It helps
speed up specs when they are run a second time. While it is not a solution for
running the whole suite faster, it does speed up the time individual specs take
when using TDD.

![gotta go fast](http://media.tumblr.com/cd4df87d3602cbbb4c0657722a9632cd/tumblr_inline_mqsgznjjcG1qz4rgp.gif)