add precise32 config.vm.box_url

for people who don't use Vagrant Getting Started anymore and don't have @mitchellh's precise32 box (really any box named "precise32")

*note: it might be a good idea to change the box name, and have config.vm.box_url set to prevent people making their own boxes named "precise32" and breaking the gauntlt quickstart, although those edge cases are probably few and far between.*