Fix reatain cycle RCTPullToRefreshViewComponentView <-> RCTScrollViewComponentView

Summary:
Changelog: [iOS][Fixed] - `_scrollViewComponentView` is set to `RCTPullToRefreshViewComponentView's` superview:
```
_scrollViewComponentView = [RCTScrollViewComponentView findScrollViewComponentViewForView:self];
```
It should be safe to make it weak.

Reviewed By: javache

Differential Revision: D36998626

fbshipit-source-id: 2130b743d181e15986cb68636d60507a986968e1