wang-1-1

Results 2 issues of wang-1-1

`UIScreen.setBrightness` will crash on iOS 13 in global thread, which preform normally before iOS 13. Crash stack: ![image](https://user-images.githubusercontent.com/50360644/62934255-a935f180-bdf6-11e9-935b-fe7c34fc4b6d.png) It will be ok if I dispatch to main queue on iOS...

https://github.com/WXGBridgeQ/SwiftPullToRefresh/commit/76393379edcb3dfc9d8eb57a9e4d162102cd1455 这个修复iOS上KVO的问题,目前最新的release 3.1未包含该commit,麻烦给master分支发布下