Vagrantfile: use the generic/ubuntu1804 box
Vagrantfile: use the generic/ubuntu1804 box

boxcutter/ubuntu1604 no longer exists.

phusion/passenger_apt_automation,phusion/passenger_apt_automation,phusion/passenger_apt_automation,phusion/passenger_apt_automation,phusion/passenger_apt_automation,phusion/passenger_apt_automation,phusion/passenger_apt_automation