Fix error "A box must be specified."

Error happen when running ```vagrant up```
Fix Refer vagrant document
http://docs.vagrantup.com/v2/providers/configuration.html
and
https://github.com/Freq-Out/airtime-appliance/commit/bb2fd4d84feb337143a2723d2ac467eb7d7deacf#diff-23b6f443c01ea2efcb4f36eedfea9089R42