use Picker from react-native-community instead of deprecated RN built-in component

fixes #120

Author: wzs

lib/countryPicker.js

@@ -1,5 +1,6 @@
 import React, { Component } from 'react';
-import { Text, TouchableOpacity, View, Modal, Picker } from 'react-native';
+import { Text, TouchableOpacity, View, Modal } from 'react-native';
+import {Picker} from '@react-native-community/picker';
 import PropTypes from 'prop-types';
 
 import Country from './country';

package.json

@@ -26,7 +26,8 @@
   "dependencies": {
     "google-libphonenumber": "^3.2.2",
     "lodash": "^4.17.4",
-    "prop-types": "^15.5.10"
+    "prop-types": "^15.5.10",
+    "@react-native-community/picker": "^1.6.5"
   },
   "peerDependencies": {
     "react-native": ">= 0.25"