Fix Vagranfile for Vagrant 1.5+

As of Vagrant 1.5:
- easy box URLs can be used to reference machines stored on Vagrant
  Cloud
- host key checking is disabled by default for the ansible provisioner
Remove user:vagrant in site.yml

- Not required, as it corresponds to the default Vagrant remote user
- This makes the playbook more generic (in case of integration out of
  Vagrant scope).
Use vagrant-cachier plugin if available
Merge pull request #1 from gildegoma/patch-1

Great, thanks for the pull request!