PHILADELPHIA — Somewhere between the corporeal torture and the attempt at lighting a fart on fire under water in “Jackass Forever,” it becomes clear that something is missing. Most of the cast, including Johnny Knoxville, Steve-O and Chris Pontius, are still there. And they’re more willing than ever to sacrifice their bodies for our entertainment, so it’s not a lack of effort. There are even some new cast members this time around — like “Loiter Squad’s” Jasper Dolphin and comedian Rachel Wolfson — leading to the most diverse group of Jackasses ever seen on screen. And it’s doing well at the bo…