fix(datepicker): fix the DatepickerIOS align bug above 0.28.0 with the removing of width

since 0.28.0, DatepickerIOS doesn't have specific width, extra code isn't need anymore.

can't be center automatic below 0.28.0. xgfe#13, xgfe#18

Author: feyy

style.js

@@ -61,7 +61,6 @@ let style = StyleSheet.create({
     right: 0
   },
   datePicker: {
-    alignItems: 'center',
     marginTop: 42,
     borderTopColor: '#ccc',
     borderTopWidth: 1