Cleanup Gemfile

As far a I understand its the common way to specify the required gem in the `gemspec` file instead of the `Gemfile`. It also looks like the `rubinius` and the `debugger` stuff belongs more in a `Gemfile.local` file, because its nothing in general, right?
`~> 0.23` implies `>= 0.23` btw

`~> 0.23` == `>= 0.23.0, < 1.0`