Remove isRequired on source prop for Android

Summary:
Makes source required across both platforms.
Closes https://github.com/facebook/react-native/pull/5845

Reviewed By: svcscm

Differential Revision: D2920250

Pulled By: vjeux

fb-gh-sync-id: 21845b601df32dc2e12a95544afa3db4c0414746
shipit-source-id: 21845b601df32dc2e12a95544afa3db4c0414746

Libraries/Image/Image.android.js

@@ -75,7 +75,7 @@ var Image = React.createClass({
       }),
       // Opaque type returned by require('./image.jpg')
       PropTypes.number,
-    ]).isRequired,
+    ]),
     /**
      * similarly to `source`, this property represents the resource used to render
      * the loading indicator for the image, displayed until image is ready to be