Errorless response with outdated GnuPG

Hey guys,

I was wondering if we could maybe put an error for outdated GnuPG? I had installed this stuff several months ago when I requested the invite for keybase, but then forgot about it and didn't use it. It was outdated and so when I tried to sign up I never got any messages. When I would run `npm install -g keybase-installer` it would give me the updated installer, but then when I would run `keybase-installer` it would just return (no output). Took me a little bit to figure out that it was just a matter of running `brew update gnupg` and now everything works great.

Thanks for your hard work! Excited to have an invite now.

Reproduce the silent return from `keybase-installer` after installing it with `npm install -g keybase-installer` and using an outdated GnuPG. Check how the installer handles GnuPG availability and version checks; done means an outdated installation produces a clear error or actionable message instead of no output.

Written by the indexing model from the issue text.