Managed "minimum" homebrew revision

This release includes an option to specify the minimum viable homebrew revision, which allows us to then depend on newer homebrew features without having to manually brew update. To override the minimum revision given with this module, you can set homebrew::repo::min_revision in hiera, or

class { 'homebrew':
  min_revision => 'deadbeef',
}

The old brew update exec resource was broken, failing because of bundler's environment, so it has been removed.