use onScroll function passed in component props for iOS

[ntorion]

Hi, I detected that onScroll function passed in props to PTRView component wasn't called only on iOS. An issue related to this problem has already been opened : https://github.com/moschan/react-native-pull-to-refresh/issues/28 This pull request fix the issue by adding the missing call.

[VicEsquivel]

I think it would be better to check if not null first right? Or will that be handled elsewhere?

const { onScroll } = this.props;
if(onScroll) onScroll(e);

[ntorion]

yes a condition is required you're right