Crash in latest master RTL code for iOS 10

iOS 10 is still officially supported by Texture, but the latest master is using API's introduced in iOS 11. This will cause any apps on iOS 10 to crash as a result.

I think @rcancro has done most of the RTL work so would be best poised to resolve it for older OS versions.

https://developer.apple.com/documentation/uikit/uicollectionviewlayout/2891099-flipshorizontallyinoppositelayout
