Build tools: Fix the `travis:js` build.

After [44359] it is impossible to not use ES6 syntax for some logic in the `Gruntfile.js`, so adjust the `esversion` setting for the `Gruntfile.js` to 6. Because the previous setting in `.jshintrc` was not compatible with setting `esversion`, set the `esversion` in the `.jshintrc` explicitly.

See #44492.


git-svn-id: https://develop.svn.wordpress.org/trunk@44361 602fd350-edb4-49c9-b593-d223f7449a82

.jshintrc

@@ -3,7 +3,7 @@
 	"curly": true,
 	"eqeqeq": true,
 	"eqnull": true,
-	"es3": true,
+	"esversion": 3,
 	"expr": true,
 	"immed": true,
 	"noarg": true,

Gruntfile.js

@@ -1,4 +1,5 @@
 /* jshint node:true */
+/* jshint esversion: 6 */
 /* globals Set */
 var webpackConfig = require( './webpack.config' );