UPdate ruby-mixlib-shellout to 2.2.7.

Release 2.2.7

* Format markdown for better rendering.
* Document more advanced usage.
* Link to the code for the complete list.
* Add logging message if attempting to kill critical process.

Release 2.2.6

* Fix regression introduced in 2.2.2 by changing CreateProcessAsUserW to use a
  :int instead of :bool for the inherit flag to fix shell_out on windows from
  a service context.

(taca)