Upgrade to Rails 4.0.5, add some useful gems

Upgrade to 4.0.5, fixes a lot of bugs and most importantly, fixes a bunch of security issues. I would have upgraded to 4.1.1, but I thought it would be better to wait for a supported version of Active Admin to be released.

I've also added [Better Errors](https://github.com/charliesome/better_errors) so my eyes stop bleeding, and [Spring](https://github.com/rails/spring), which seems to be the new Zeus.