Update sync script to the latest from @dgoodlad

As gisted at https://gist.github.com/dgoodlad/34f429d1b84a471933d3, and
linked from https://github.com/boxen/puppet-homebrew/issues/60, this is
the latest sync script used by Github's our-boxen. It uses the newer
aws-sdk-core gem, and apparently actually works.