Update ImagePickerManager.m (#573)
Update ImagePickerManager.m (#573)

modify the way to get root view controller.
marcshilling/react-native-image-picker,marcshilling/react-native-image-picker,react-community/react-native-image-picker,react-community/react-native-image-picker,react-community/react-native-image-picker,react-community/react-native-image-picker,marcshilling/react-native-image-picker,marcshilling/react-native-image-picker