gtk2 rbgdkdraw.c: drop GTK_CHECK_VERSION(2,8,0)
gtk2 rbgdkscreen.c: drop GTK_CHECK_VERSION(2,8,0)
gtk2 rbgtkimage.c: drop GTK_CHECK_VERSION(2,8,0)
gtk2 rbgtkscrolledwindow.c: drop GTK_CHECK_VERSION(2,8,0)
gtk2 rbgtkaction.c: drop GTK_CHECK_VERSION(2,8,0)
gtk2 rbgtkdisplay.c: drop GTK_CHECK_VERSION(2,8,0)
update READMEs