use pessimistic version constraint

- this is compatible with gem versions from 11.6.0 with minimal
  changes.
- mixlib-shellout differs in windows-versus-unix due to version pinning
  in windows-pr picking up old mixlib-shellout on windows, this is
  not fixed, and left similar to 11.6.0
- gem versions should be levelled up for 11.8.x and beyond
- newly built 11.6.0 will pick up new mixlib-config 2.0 which is
  a delta from the downloadable omnibus builds, 11.6.2 will not
  pick up mixlib-config 2.0
Merge branch 'opscode-11.6-stable' into 11.6-stable

Conflicts:
	chef.gemspec