A teenage gunman is in custody after a weeks-long crime spree that included back-to-back kidnappings in June, authorities say.Dominick Higgins, 18, and an accomplice in a hooded sweatshirt ambushed a man in the early morning hours of June 24 outside the Courtesy Diner on South Kingshighway, police say. The 34-year-old victim later told investigators that he was in the parking lot when two robbers pressed a gun to the back of his head and told him to get in his car.…