Renamed gtkmm.spec -> gtkmm24.spec
    
    Fixed up spec file name to match git repo name. Fedpkg makes the
    assumption that these names match up and fails to interact with the
    lookaside cache if they don't.
    
    Renaming the spec file as suggested by Jesse Keating.
    
    https://fedorahosted.org/fedora-packager/ticket/75