fix ohai path

need to install the gem with -n /opt/chef/bin before doing the
bundle install or else ohai winds up in /opt/chef/embedded/bin
remove the windows gem before installing the unix one

- we need to avoid picking up the mingw gem in this software def
- the windows installs use chef-windows.rb in the omnibus-chef project
  (which is probably a bug), but that means this file is unix-only.