fix storage test

src/storage/gcs.js

@@ -40,7 +40,7 @@ const assert = require('assert'),
 let GCS = require('@google-cloud/storage').Storage;
 
 // test only
-var originalGCS;
+let originalGCS;
 function mockInject(mock) {
     originalGCS = GCS;
     GCS = mock;
@@ -54,7 +54,7 @@ function mockRestore() {
 function getBucket(apiConfig) {
     assert.strictEqual(typeof apiConfig, 'object');
 
-    var gcsConfig = {
+    const gcsConfig = {
         projectId: apiConfig.projectId,
 
         credentials: {
@@ -95,7 +95,7 @@ function upload(apiConfig, backupFilePath, sourceStream, callback) {
         callback(null);
     }
 
-    var uploadStream = getBucket(apiConfig).file(backupFilePath)
+    const uploadStream = getBucket(apiConfig).file(backupFilePath)
         .createWriteStream({resumable: false})
         .on('finish', done)
         .on('error', done);