Add json format version to the json output

lib/output/json.js

@@ -21,6 +21,9 @@ JSONOutput.prototype.onPage = function(page) {
         // Delete some private properties
         delete json.config;
 
+        // Specify JSON output version
+        json.version = '2';
+
         return that.writeFile(
             page.withExtension('.json'),
             JSON.stringify(json, null, 4)