Update sharedElementTransition.ios.js (#1163)

Fix missing import of PropTypes throwing an error on iOS

src/views/sharedElementTransition.ios.js

@@ -1,4 +1,4 @@
-import React, {Component} from 'react';
+import React, {Component, PropTypes} from 'react';
 import {
   View
 } from 'react-native';