Navigator: Changed transitioner background color to 'transparent'

Summary:
... for setting the transition background color between scenes. For issue #563
Closes https://github.com/facebook/react-native/pull/654
Github Author: Josh Levine <josh.levine1@gmail.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/CustomComponents/Navigator/Navigator.js

@@ -88,7 +88,7 @@ var styles = StyleSheet.create({
   },
   transitioner: {
     flex: 1,
-    backgroundColor: '#555555',
+    backgroundColor: 'transparent',
     overflow: 'hidden',
   }
 });