Switch PropTypes to external package

Since React v15.5 PropTypes has been moved out into an external
`prop-types` library. See
https://reactjs.org/docs/typechecking-with-proptypes.html for
more details.

ActionButton.js

@@ -1,6 +1,5 @@
 import React, {
   Component,
-  PropTypes,
 } from 'react';
 import {
   StyleSheet,
@@ -10,6 +9,7 @@ import {
   TouchableOpacity,
   TouchableWithoutFeedback,
 } from 'react-native';
+import PropTypes from 'prop-types';
 import ActionButtonItem from './ActionButtonItem';
 
 const alignMap = {

ActionButtonItem.js

@@ -1,13 +1,13 @@
 import React, {
   Component,
-  PropTypes,
 } from 'react';
 import {
   StyleSheet,
   View,
   Animated,
   TouchableOpacity,
 } from 'react-native';
+import PropTypes from 'prop-types';
 
 export default class ActionButtonItem extends Component {
 

package.json

@@ -27,5 +27,8 @@
   "bugs": {
     "url": "https://github.com/geremih/react-native-circular-action-menu/issues"
   },
-  "homepage": "https://github.com/geremih/react-native-circular-action-menu"
+  "homepage": "https://github.com/geremih/react-native-circular-action-menu",
+  "dependencies": {
+    "prop-types": "^15.6.1"
+  }
 }