By Denny Dyroff, Staff Writer, The Times If it seems as if “Riverdance” has been around forever — it’s because it has been around forever. If you’re not of the age to drink legally in Pennsylvania or Delaware, then you weren’t even born yet when “Riverdance” made its stage debut in London Its popularity never seems to wane. Like the Energizer Bunny and Betty White, “Riverdance”...