Re-enable Ruby tests, pegging to RSpec 2.x

Hoping this will fix https://github.com/facebook/watchman/issues/41

Copy-pasta-ing my comment from that issue:

> I believe this is caused by the move to RSpec 3.0; I believe we can fix this by specifying a desired RSpec version in the gemspec. (We could also try to tweak the tests so that they pass in RSpec 2.x or 3.x, but that's going to be a game of whack-a-mole, possibly breaking whenever RSpec decides to make upstream changes.)