Move fuubar gem into development group
rubinius name on travis rvm matrix has changed

Started seeing this earlier in the week, and just noticed a discussion
on the travis-ci issues list today: https://github.com/travis-ci/travis-ci/issues/1641

Seems like using rbx and rbx2 are the two supported versions right now.
Ideally rbx-19mode would still work so we can explicitly say which version
of ruby under rubinius we want, but alas.