Hardcode ruby devkit into Windows customization

Fixes chef-dk#100 by adding hardcoding the necessary bits for the devkit
into the operating_system.rb file that we currently create to ensure gems
are installed into the users directory on Windows. Also reverts 5b99247,
as it proved only half of a solution, the other half being complicated
balancing of build-order to make sure rubygems-customization happened at
exactly the right time (before devkit, but not before bundler, at least).