fix(.gitignore): hide local vagrant config.rb from version control

A local `config.rb` file overrides items from the `Vagrantfile`, but we forgot to exclude it from version control in #1881 when aligning with [coreos-vagrant](https://github.com/coreos/coreos-vagrant).

See https://github.com/coreos/coreos-vagrant#configuration and https://github.com/coreos/coreos-vagrant/blob/master/config.rb.sample for more info.