Log Message:
Update ruby-mixlib-shellout to 2.1.0.

Release 2.1.0

    * BackSlasher: login flag now correctly does the magic on unix to
      simulate a login shell for a user (secondary groups, environment
      variables, set primary group and generally emulate su -).
    * went back to setsid() to drop the controlling tty, fixed old AIX
      issue with getpgid() via avoiding calling getpgid().
    * converted specs to rspec3