NEW YORK — Diet Coke is getting a makeover to try to invigorate the sugar-free soda’s slumping sales. Coca-Cola Co. said Wednesday it’s adding a slimmer, 12-ounce Diet Coke can, updating the logo and offering the 35-year-old drink in four new flavors, including mango and ginger lime. The taste of the plain Diet Coke will stay the same, the Atlanta-based company said. Diet Coke sales have fallen as more people switch to other low-calorie drinks, such as flavored fizzy water.