Ref #20 - Removing Rubinius build on travis-ci

- I've reduced the number of Ruby versions that `how_slow` is built against
- I'm certain that `how_slow` will soon be used in at least one `ree` based app,
  so that's staying
- Ruby 1.9.2 and 1.9.3 are commonly in use, so they stay
- Ruby 2.0 support is important for the future, so it stays

I will readd Travis builds as requested by users.