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>