version: fix package and tests

Introduce `mustOsVersion`, which replaces `osVersion`, and panics if the version cannot be determined.

`osVersion` now returns an error value if it cannot determine the series.

Fix the tests so they actually _compile_ and _pass_ on OSX.
It may be the case that it's currently broken, but that doesn't mean we should document the usage of the function to match the bug. It seems pretty reasonable to me to have a function that returns a list of supported series, and for it to work regardless of where it's called from.

I think this is a case of: BUG this thing only works on Ubuntu.