When version is nil, (version <==> other.version) also returns nil
and then the nill object doesn't have a nonzero? method.
I'm not sure what package put me into this state, but, this fix repaired
my environment.
Also, there is probably a more ruby-esque way to do this, but, I'm not a
ruby expert ;-)
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.