Cleanup for future development

[chesterbr]

Tentative list of things that should be done to bring this project back to life for 2017.

• [x] Ensure we run on Ruby 2.1.3 and latest Ruby 2.1.4.
• [x] Ensure we run on latest JRuby (code and specs)
• [x] ~~Migrate the Gemspec back into a Gemfile (take JRuby in consideration)~~ (decided against it; noticing that optcarrot is also built as an unpackaged gem)
• [ ] Have a better command-line interface (replace bootstrap scripts)
• [ ] Provide at least one example ROM (2048 2600?)
• [x] Bump to latest RSpec (consider Minitest + some sugar)
• [ ] Consider alternatives to Gosu (or ensure we can easily install it)
• [ ] Add some CI
• [ ] Update / enhance / integrate #9 (Opal).