Move the vagrant xenial base image to the Vagrant Atlas repository

To better handle box versioning and updates.

Vagrantfile

@@ -4,7 +4,6 @@
 Vagrant.configure(2) do |config|
   # TODO: Switch back to the official box once it is released
   config.vm.box = 'opencraft/xenial64'
-  config.vm.box_url = 'http://opencraft.com/files/vagrant-opencraft-xenial64.box'
   config.vm.synced_folder '.', '/vagrant', disabled: true
   config.vm.synced_folder '.', '/home/vagrant/opencraft'