Use precise64 as box for all fixtures

This PR sets all fixtures to use `precise64`, so that new developers
would not have to download more boxes to run the specs than they need
to.  We also set a URL for `precise64`, so Vagrant can download the box
if it hasn't yet.

I settled with `precise64` as it is the current LTS, and because there
is a box for it at `http://files.vagrantup.com`.