Cleanup for future development

[chesterbr]

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

• Ensure we run on Ruby 2.1.3 and latest Ruby 2.1.4.
• Ensure we run on latest JRuby (code and specs)
• 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?)
• 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 Running ruby2600 in Opal #9 (Opal).