fix(PickerWindows): Fix overlooked change from recent refactoring (#1221)
fix(PickerWindows): Fix overlooked change from recent refactoring (#1221)

Recent refactoring eliminated deprecated `React.PropTypes` in favor of `prop-types` module. This change was overlooked during the refactoring (cc @jmcginty).

Fixes #1220
lbochenek/react-native-windows,lbochenek/react-native-windows,lbochenek/react-native-windows,lbochenek/react-native-windows