Triage Travis CI configuration

  * Use latest rubies for travis
  * Switch to travis-handled rbx version; add dependent gems
  * Use latest appraisal gem that supports Bundler platforms
  * Update appraisal gemfiles to use latest appraisal
  * Add fragile rubies to allowable failures in travis-ci

Rubinius support in Travis CI was really hard to nail down, and now it looks like `appraisal` is doing some weird things on Travis with `rake`. I'm thinking we should allow rbx failures for now. What do you think @stefanpenner?