Provide our own bootstrap script for Salt

By default, Vagrant goes to bootstrap.salt.org to get the latest bootstrap script for Salt to make sure that all packages are avaiable for whatever OS it is required on. This URL now 302 redirects to the latest script at https://raw.github.com/saltstack/salt-bootstrap/stable/bootstrap-salt.sh, which returns a 404 every once and a while when called by curl. Whenever it returns a 404, things bail and provisioning comes to a stop.

Luckily, Vagrant allows us to specify our own bootstrap file through salt.bootstrap_script. I've copied the current master into this repo and specified that as the default. Things don't appear to change too often, so we won't have to worry much about getting out of date. I've subscribed to upstream updates to make sure I am aware of changes that happen.