Return value for add

Return the incremented counter value for `Ratelimit#add`. This is useful if want to ensure that the ratelimit is not exceeded in by parallel processes. Previously add returned an array of return values for all pipeline commands, but only `HINCRBY` has a meaningful value.

Gemfile/Travis configuration was updated to fix Rubinius build. Adding 2.1.1 caused additional test failures, which were solved by updating gems.