Add assets group to Gemfile, update to Rails 4.0.5 for security patch, add Spring for faster tests

This PR will just organize our Gemfile a bit by adding the assets group. Also, Rails 4.0.5 is now recommended for a security patch (note: this is not Rails 4.1 which brings about many changes).

Also, this gem adds [Spring.](https://github.com/rails/spring) It preloads the rails environment for testing, making our tests run ~3 seconds faster. Note: you will have to run ```spring cucumber``` or ```spring rspec``` to see the results.

On top of that, I did a little linting on our Guardfile to clean spork-guard, and account for spring. If you use Tmux, there are now notifications automatically in the terminal if your tests pass/fail.

Wewt.