paginationWaitingView is not required

[geminiyellow]

@iwater

when i copy the example from README, got this warning,

Warning: Failed prop type: The prop `paginationWaitingView` is marked as required in `GiftedVirtualizedList`, but its value is `undefined`.

yes, in here, it is isRequired,

react-native-infinite-virtualized-list/src/GiftedVirtualizedList.js

Line 9 in 962cd4d

paginationWaitingView: PropTypes.func.isRequired,

but, where it used,

here:

react-native-infinite-virtualized-list/src/InfiniteVirtualizedList.js

Line 14 in 962cd4d

paginationWaitingView: PropTypes.func,

and default:

react-native-infinite-virtualized-list/src/InfiniteVirtualizedList.js

Lines 18 to 20 in 962cd4d

	static defaultProps = {
	paginationWaitingView: noop,
	}

so, i think you should remove the isRequired from API.