[JSHint] Use "mocha" option instead of specifying globals explicitly

From v2.5.0, JSHint supports the predefined functions of [Mocha](http://visionmedia.github.io/mocha/) with [`mocha` option](https://github.com/jshint/jshint/blob/20e2bee28242fa627e3a98087dda8713850941a9/src/jshint.js#L144).
See jshint/jshint#1597 for details.