Changed button view topBorderWidth to the hairlineWidth
Changed button view topBorderWidth to the hairlineWidth

This change addresses a render issue on iOS devices with high pixel density (iPhone 6+ & 7+).
The underlying issue is documented here https://github.com/facebook/react-native/issues/12401
markuswind/react-native-select-input-ios,markuswind/react-native-select-input-ios,markuswind/react-native-select-input-ios,markuswind/react-native-select-input-ios