Bump versions for Travis CI (and remove rbx)
Bump versions for Travis CI (and remove rbx)

Bump all of the Ruby versions for Travis CI to the latest available.
Additionally, remove Rubinius from the test list. I allowed failures for
it once they switched to their (stupid) new non-semver versioning
scheme, and now it seems that rvm just doesn't want to install it at
all. So, no point in wasting my time with it I guess.

ahamlinman/peribot