Fix package versioning, fix apt repos

Installing 'latest' is a bit of an anti-pattern.

This changes the default to 'installed', fixes some end-of-line spaces, and fixes the apt install so it'll install in one run (previously it was executing apt-get update before the apt-key command, leading to problems with apt not installing because it can't find the keys).
You mentioned using latest is a bit of an anti-pattern. For the fact this is the module literally to install and manage the duo_unix package, why wouldn't I expect people to maintain latest (by default)?

I'm genuinely just not sure when/if this is actually a "rule" for modules these days (or always has and I was just blissfully unaware).