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` 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