Set config.vm.box_url in Vagrantfile and fix add_gittip_user.sql

Vagrant users shouldn't have to manually do a 'vagrant box add' in order to get the precise64 box installed.  Adding this config option to the Vagrantfile will make Vagrant automatically fetch the box on 'vagrant up' if the user doesn't already have a copy.

I've also fixed and tested the add_gittip_user.sql file.  A 'gittip' Postgres superuser is now automatically created on 'vagrant up'.