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
revise travis build so that we can run osx and linux

This also pulls in hgwatchman so that we can run its test suite.
I've disabled this for the moment because it takes a while on osx
and errors out :-/

We need to resolve that, but in the meantime, there's no reason not to
proceed with the build fixed up.

Also note: this turns off valgrind on travis builds too; we can revisit
and enable in a follow-on diff