Valid platforms: MacOSX + macOS (2016)

[roolo]

In 2016 Apple changed naming to macOS -- https://en.wikipedia.org/wiki/MacOS#macOS . I think, there should be new valid platform in Ansible which would reflect this change.

[chouseknecht]

Thanks for pointing this out!

Added 'macOS Sierra'. Going forward we'll use 'macOS' as the platform, and the name (e.g. Sierra, El Capitan, etc) as the release. This seems to line up with Apple's web site.