[changed] Change Navigation to return the result of goBack()

Upgrade path: The goBack function now returns true (on successful back) or false (no react-router history to navigate back to), previously the return from goBack was always undefined.

modules/Navigation.js

@@ -64,7 +64,7 @@ var Navigation = {
    * Transitions to the previous URL.
    */
   goBack: function () {
-    this.context.goBack();
+    return this.context.goBack();
   }
 
 };