Stop using deprecated method free_colors() (#652044)

Gtkmm releases 2.7.1 in July 2005 and 2.8 in September 2005 deprecated
Gdk::Colormap::free_colors() and replaced it with
Gdk::Colormap::free_color().  The oldest distribution currently
supported is RHEL / CentOS 5.9 with gtkmm 2.10 so just unconditionally
replace.

    ANNOUNCE: gtkmm 2.7.1
    https://mail.gnome.org/archives/gnome-announce-list/2005-July/msg00025.html

    ANNOUNCE: gtkmm and glibmm 2.8
    https://mail.gnome.org/archives/gnome-announce-list/2005-September/msg00026.html

Bug #652044 - uses deprecated APIs