travis: remove "fast" from "makecache fast"
    
    dnf makecache does not support the "fast" keyword in its
    makecache subcommand in Fedora 26.
    
    https://pagure.io/freeipa/issue/6874
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>