Decimated by injuries and wrecked by poor performance, the Rockies’ starting rotation needs to be rescued. But as the second half of the season opens Friday night vs. the Yankees at Coors Field, the cavalry is not rushing in. There is no knight in shining armor riding in on his white steed from the minor leagues. The Rockies did make a roster addition Thursday, signing free-agent right-hander Chris Flexen to a minor-league deal.