Add string as possible propType for 'systemIcon'

Summary:
Fixes #2361
Closes https://github.com/facebook/react-native/pull/4523

Reviewed By: svcscm

Differential Revision: D2718568

Pulled By: androidtrunkagent

fb-gh-sync-id: a15c94acaac899dfdaeb397c8c764053d1430854

Libraries/Components/TabBarIOS/TabBarItemIOS.ios.js

@@ -52,7 +52,10 @@ var TabBarItemIOS = React.createClass({
     /**
      * A custom icon for the tab. It is ignored when a system icon is defined.
      */
-    icon: Image.propTypes.source,
+    icon: React.PropTypes.oneOfType([
+      React.PropTypes.string,
+      Image.propTypes.source,
+    ]),
     /**
      * A custom icon when the tab is selected. It is ignored when a system
      * icon is defined. If left empty, the icon will be tinted in blue.