replace experimentalObjectRestSpread with ecmaVersion: 2018

https://github.com/tc39/proposal-object-rest-spread has become a part
of the ECMAScript standard as of version 2018 (also known as ECMAScript
9).

experimentalObjectRestSpread will be removed in the future from eslint.

es6.js

@@ -20,8 +20,6 @@ module.exports = {
         es6: true
     },
     parserOptions: {
-        ecmaFeatures: {
-            experimentalObjectRestSpread: true
-        }
+        ecmaVersion: 2018
     }
 };