Only support Ruby 1.9.3+

Okay, removed all references to Ruby 1.8, and associated cruft that is no longer necessary now that we can assume 1.9+, like explicitly requiring rubygems. I've added Ruby 2.1 to the Travis build matrix, and while I was at it, parallelized bundler's install process on Travis, so we should see a nice speed-up from that.

I'll start bumping some of the other libraries, now that we're not held back by 1.8 support.