fix ostree repo location
    
    problem: fedora atomic host images are including an old
    ostree tree (24.39 from 2016-06-15 09:57:04)
    
    cause: The ostree repo specified in this file at:
    https://kojipkgs.fedoraproject.org/compose/atomic/24/
    was last updated at 2016-06-15 09:57:04.
    
    Fedora Atomic 23 used the location:
    https://kojipkgs.fedoraproject.org/mash/atomic/23/.
    
    The ref at the similar
    https://kojipkgs.fedoraproject.org/mash/atomic/24/
    was last modifed 2016-07-24. This seems to be the correct
    location. Or, if the switch from mash to compose is right,
    then we need to ensure that the compose location is updated
    / kept up to date.