gtk3: add GTK_CHECK_VERSION(3,6,0)
gtk3: add GTK_CHECK_VERSION(3,6,0)

Because GtkLevelBar is since GTK 3.6.

kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2