tools: update ESLint to 2.9.0

ESLint 2.9.0 fixes some minor bugs that we have been experiencing and
introduces some new rules that we may wish to consider.

PR-URL: https://github.com/nodejs/node/pull/6498


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>