tools: update ESLint to version 2.1.0

tools: update ESLint to version 2.1.0

Update ESLint to 2.1.0. ESLint has a number of potentially-useful new
features but this change attempts to be minimal in its changes.
However, some things could not be avoided reasonably.

ESLint 2.1.0 found a few lint issues that ESLing 1.x missed with
template strings that did not take advantage of any features of
template strings, and `let` declarations where `const` sufficed.

Additionally, ESLint 2.1.0 removes some granularity around enabling ES6
features. Some features (e.g., spread operator) that had been turned off
in our configuration for ESLint 1.x are now permitted.

PR-URL: https://github.com/nodejs/node/pull/5214
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: jbergstroem - Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Myles Borins <myles.borins@gmail.com>