Convert react-native-github/Libraries to let/const

Reviewed By: sahrens

Differential Revision: D7956042

fbshipit-source-id: 221851aa311f3cdd6326497352b366048db0a1bb

Libraries/Sample/Sample.ios.js

@@ -3,13 +3,13 @@
  */
 'use strict';
 
-var NativeSample = require('NativeModules').Sample;
+const NativeSample = require('NativeModules').Sample;
 
 /**
  * High-level docs for the Sample iOS API can be written here.
  */
 
-var Sample = {
+const Sample = {
   test: function() {
     NativeSample.test();
   }