Add better_errors to the default Gemfile

Hi!

This pull request adds my [better_errors](https://github.com/charliesome/better_errors) gem and its optional dependency [binding_of_caller](https://github.com/banister/binding_of_caller) (now working on Ruby 2.0.0 thanks to the new debug_inspector API) to the default Rails 4 Gemfile.

Better Errors has been quite popular with Rails developers since release, so I figure it's a good fit for inclusion in the default set of gem dependencies.