They just announced on SummerSlam that the PPV is leaving Los Angeles after six years, heading to "NY/NJ" next year (which means northern New Jersey).
The announcement featured a lot of shots of Brooklyn and nothing featuring Madison Square Garden, so it would appear the event is being held at The Barclays Center in Brooklyn, NY.
*UPDATE*
The early word is that WWE is targeting the IZOD Center in East Rutherford, New Jersey as the home for next year's SummerSlam. It's not a done deal yet, but that is where the company is currently targeting.