tools: enable ES2016 syntax support in ESLint

This allows us to use the exponentiation operator.

PR-URL: https://github.com/nodejs/node/pull/9218
Ref: https://github.com/nodejs/node/pull/9208#issuecomment-255309920
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

.eslintrc

@@ -2,6 +2,9 @@ env:
   node: true
   es6: true
 
+parserOptions:
+  ecmaVersion: 2016
+
 rules:
   # Possible Errors
   # http://eslint.org/docs/rules/#possible-errors