Use powershell_script in chef 11.6+

Closes #15
Cleaned up rubocop errors
README: Updated to require 11.6+

Did some other minor formatting tweaks while I there.  `pandoc`
makes it easy.
Handle Chocolatey version 0.9.8.24+

Chocolatey moved things around in version 0.9.8.24

This should be mostly future proof. *crosses fingers*