Test on Travis and a couple Rubinius-related tweaks.

Hi,

I've add testing on Travis (build for this PR hasn't popped up in Travis yet) and a few tweaks for Rubinius.

As I understand it, correct form is to put rake in the Gemfile for test or development. But I am not a Bundler expert. It helps with Rubinius because we encourage people not to depend on global binstubs and instead exec things via `rbx -S foo`, which is especially nice with our -G flag that avoids bundle exec.