Schedule builds from last known SRPM
    
    Since Fedora 24 branching, rawhide (f25) doesn't inherit from branched
    (f24) any longer, see [1,2].  This means that packages that failed to
    build during mass rebuild won't have SRPM tagged into f25 and Koschei
    wouldn't be able to schedule scratch builds for them.  With this
    change, Koschei will run builds from last known SRPM (possibly from
    Fedora 24).
    
    This change also makes submitting builds a bit faster as two Koji
    queries are avoided.
    
    [1] https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/message/GXVKAAFY3EDQMNGCSJPNQRSLEONCB2MH/
    [2] http://koji.fedoraproject.org/koji/taginfo?tagID=f25