Support Chocolatey version 0.9.8.24+

Chocolatey moved things around in version 0.9.8.24

* Added only_if checks for upgrading chocolatey itself.
* Use the `ChocolateyInstall` Environment variable for the install
  location (thanks to feedback from issue #16). We no longer make
  assumptions about where Chocolatey is installed.
* Use powershell_script. This is only available in chef 11.6+
* Cleaned up rubocop errors
* Updated (and reflowed via `pandoc`) README

Closes #15