check if this._navBar has immediatelyRefresh method

Summary:
`Navigator` throws an error if using a custom `NavigationBar` component that does not have a `immediatelyRefresh` method present.
Closes https://github.com/facebook/react-native/pull/9438

Differential Revision: D3838928

Pulled By: mkonicek

fbshipit-source-id: 74d62ef09e179f457a4b14f8537dfaf0d4697322

Libraries/CustomComponents/Navigator/Navigator.js

@@ -495,7 +495,10 @@ var Navigator = React.createClass({
       transitionQueue: [],
     }, () => {
       this._handleSpringUpdate();
-      this._navBar && this._navBar.immediatelyRefresh();
+      var navBar = this._navBar;
+      if (navBar && navBar.immediatelyRefresh) {
+        navBar.immediatelyRefresh();
+      }
       this._emitDidFocus(this.state.routeStack[this.state.presentedIndex]);
     });
   },