Our zsh configuration depends on a few features that require at least
version 3.1.6 of the shell. Personally, I think requiring version 3.1.7
should be even safer (as 3.1.6 introduces quite a *lot* of new features).

I suggest we check for $ZSH_VERSION to be at least 3.1.7 and higher,
and warn loudly, if that check fails (so nobody wonders why it fails
with versions 3.0.x).

Comments? If none, expect me to take a shot at this around next weekend.

Regards, Frank