Now Ruby comes with its own bundler let's favour using it when we can
over requiring a system one be installed.
This avoids needing to have anything in `~/.gem` again.
I am somewhat optimistic this may help with #6579 but it's useful
by itself.
The main class you should look at is the {Formula} class (and classes linked from there). That's the class that's used to create Homebrew formulae (i.e. package descriptions). Assume anything else you stumble upon is private.