attemptFallback should be invoked on each node rather than on each child

[jverkoey]

  for (MDMViewControllerTransitionContextNode *child in _children) {
    [child attemptFallback];

    [child start];
  }

should be

  [self attemptFallback];

  for (MDMViewControllerTransitionContextNode *child in _children) {
    [child start];
  }

otherwise composed transitions will not attempt to fallback.