build: bump GTK+ required version
build: bump GTK+ required version

3.1.18 is now required for pathbar theming and GtkThemingEngine's
gtk_render_line() implementation which we rely upon now.

magcius/gnome-themes-standard,Distrotech/gnome-themes-standard,ChrisCummins/gnome-themes-standard,cosimoc/gnome-themes-standard,ChrisCummins/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,Distrotech/gnome-themes-standard,rgcjonas/adwaita-semidark,cosimoc/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,ChrisCummins/gnome-themes-standard,magcius/gnome-themes-standard,rgcjonas/adwaita-semidark,rgcjonas/adwaita-semidark,magcius/gnome-themes-standard,rgcjonas/adwaita-semidark,cosimoc/gnome-themes-standard,ChrisCummins/gnome-themes-standard,cosimoc/gnome-themes-standard,ChrisCummins/gnome-themes-standard