Enforce Ruby 1.9.2+ requirement

I was just graced with a Ruby 1.8 project and learned the hard way that while git-up installs just fine under Ruby 1.8.7, posix-spawn 0.3.9 (itself a grit dependency) blows up.

This enforces a Ruby 1.9.2+ requirement. I checked that git-up does work on 1.9.2-p330 and 1.9.3-p547; 1.9.1-p431 doesn’t install for me on Ubuntu 14.04 and I don’t think Ruby 1.9.1 userbase size warrants further experiments.

Thanks so much for git-up!