Updated to work with Ruby 2.0

This is a nice little game you have here and could be helpful for many people to see as an example, so I took the time to update it to work with the latest version of Ruby.

* Updates Gosu to latest version
* Removes ruby-debug dependency
* Adds `.ruby-version` file for RVM and rbenv
* Cleans up custom `require_local` to use standard `require_relative` method