Update mixlib-shellout to recover from the yanked 1.6.0 version.

/cc: @schisamo 

Also permission to if possible cherry-pick otherwise ninja-push to all *-stable branches:

* chef-11-stable
* chef-12-stable
* chefdk-0-stable
* chef-provisioning-12-stable

Not sure if we need chef-provisioning-12-stable. Looks like angrychef, chef-container tags along chef for the ride. Any specific reason to have chef-provisioning-stable branches?