Refactor on_ruby helper to handle Ruby 2.0

While working on [fixing the build](https://github.com/infertux/simplecov/tree/fix_build), I came across the [on_ruby](https://github.com/colszowka/simplecov/blob/master/test/shoulda_macros.rb#L6-L10) helper. It seems a bit brittle since it doesn't run all tests for Ruby 2.0 assuming it is under 1.8.
Here's a quick refactoring to handle Ruby 2.0, please let me know if that makes sense so I could try to fix the build for 2.0.
I'm a bit confused about lines like `on_ruby '1.8', '1.9' do` - what version did you intend to exclude here? Thanks!