Build on Ruby 1.9.3 and 2.0.0

Previously Rails was just building on Ruby 2.0.0. This pull request makes it build on 1.9.3 as well.

I've had to skip over the memcached tests because they're currently broken. These failures are probably more indicative of us not setting up the CI environment for Rails properly, rather than any real broken-ness.

cc @github/rails