Use Arch Linux Vagrant box (take 2)

Can't test `ping` because of a Vagrant bug
<https://github.com/mitchellh/vagrant/issues/4288> (closed but it seems not
yet fixed).

Gem installs are borked (the executable is only readable by root), so we
have to install as vagrant.

<https://github.com/terrywang/vagrantboxes/blob/master/archlinux-x86_64.md>
via <https://wiki.archlinux.org/index.php/Vagrant#Base_Boxes_for_Vagrant>.