Fixes regression and improves playbook

Fixes regression introduced by #1
Simplified installation : vagrant machines are setup so no special
options are required at the CLI or in hosts file.
Improved README so it doesn't appear broken on GH.
Increased VM memory
Changed rubies to more up to date versions
Fixes merge conflict from unpulled upstream
Fixes vagrantfile

Used hack so provisionning happens in parallel
Used another hack so manual hosts pinging is not necessary anymore
Ansiblified checking VM for installed ruby versions
Added prereqs section with links