Better usage of crowbar machines API

As we already can add an optional path to the show action of the
machines command i have replaced the JSON parsing stuff with this simple
call.
Install rubygems and rubygem-json

To make the update process simple i am installing rubygems and
rubygem-json manually for ruby 1.8 to be able to execute all the ruby -e
calls that require rubygems and json.
Merge pull request #149 from SUSE-Cloud/feature/better-api-usage

Rails 4 update requirements