Parameterize Ruby versions: adds rspec and #get_choice_from_list

It's fairly well tested, but it's a pretty big refactor so please look it over.

* Added rspec
* Added Railsbricks::RUBY_VERSIONS and Railsbricks::DEFAULT_RUBY (with specs)
* Added Menu#get_choice_from_list (with specs)
* Refactored WizardMenu to use #get_choice_from_list and the constants above (no specs)
* Added some specs for a hypothetical Menu#get_yes_no_choice

Sorry I couldn't figure out how to break this commit up a bit.