add missing gem dependencies

* need rdoc for dev
* need json for dev and regular usage

In a clean bundle environment on F20, `bundle exec rake -T`
fails unless the bundle includes the deps.
dont reference build script etcd has removed it
Merge pull request #46 from ranjib/build_fix

(chore) Fix travis build
Merge pull request #43 from jumanjiman/deps

add missing gem dependencies