Re-enable Ruby tests, pegging to RSpec 3.0

These started failing when RSpec 3.0 came out, so adapt the specs to
make the tests pass under RSpec 3.0, and peg the dependency to that
revision so that we don't have to fix this again in the future.

Note that we have to run `bundle install` manually because Travis will
only do it for us automatically if it detects a Gemfile in the root
level of the repo:

http://docs.travis-ci.com/user/languages/ruby/

Sample failure:

https://travis-ci.org/facebook/watchman/jobs/29939087

Additionally, ensure we never try a `bundle exec` without first
confirming that the `bundle` is present:

https://travis-ci.org/facebook/watchman/jobs/29939419

Fixes: https://github.com/facebook/watchman/issues/41