`brew cask list` supports the `--full-name` flag which will include the tap
name for casks not part of the core caskroom/cask tap. For example, if
cask "foo-beta" is installed from the caskroom/versions cask, `brew cask
list --full-name` will report the name as "caskroom/versions/foo-beta".
Homebrew's actually ended up using a fair few gems. While we want to
avoid Bundler at runtime (and this PR still does that, in fact uses
Bundler even less at runtime than it did before) writing our own version
to use at build-time seems redundant.
