Upgrade to WKWebView on iOS

[chrisgarrity]

https://developer.apple.com/news/?id=12232019b

The UIWebView interface has been deprecated for some time. Apple will stop accepting app updates that use UIWebView in December of this year.

Note that we originally didn't update because it required iOS8, and ScratchJr was still supporting iOS 7 at the time. We have since set the minimum iOS required to 8.0, so that's no longer a concern.

[Mad-hu]

When can get a new version?

[chrisgarrity]

Prepare for next ScratchJr release