call type_register only if needed [2.8+ does not need it]

src/cell_renderer_image.py

@@ -121,4 +121,5 @@ class CellRendererImage(gtk.GenericCellRenderer):
 					self.get_property('ypad'))
 		return x_offset, y_offset, calc_width, calc_height
 
-gobject.type_register(CellRendererImage)
+if gtk.gtk_version < (2, 8, 0) and gtk.pygtk_version < (2, 8, 0):
+	gobject.type_register(CellRendererImage)