jshintrc: use type Number for esversion (#9049)

no issue

- refs http://jshint.com/docs/options/
- the value is a Number, not a String
- e.g. if you use template strings, jshint complains

.jshintrc

@@ -23,5 +23,5 @@
     "predef": [
         "-Promise"
     ],
-    "esversion": "6"
+    "esversion": 6
 }