remove unused function type
move windows specific code to windows file
introduce mustOsVersion
Fix tests for osx
Fix windows version
wording
rename mustOsVersion to mustOSVersion
tweak
Merge pull request #195 from davecheney/118-fix-os-version-some-more

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.