Use jshint v2.0.0 or better. ES5 is now default.

JSHint complains loudly when using the es5 option with
version 2.0.0 and later. Remove es5 from our .jshintrc
and require at least v2.0.0 or better to jshint for es5.

See also: https://github.com/jshint/jshint/commit/c001e7c6d7ab8bf6fdd940935d14360896121eea