[git] ignore generated boxes
[git] ignore generated boxes

englishm/vagrant-xen,englishm/vagrant-xen,sjfloat/mirage-vagrant-vms,yifanz/mirage-vagrant-vms,mirage/mirage-vagrant-vms,sjfloat/mirage-vagrant-vms,yifanz/mirage-vagrant-vms,mirage/mirage-vagrant-vms