Update JSHint to the latest version.

JSHint 2.5 was like a major release and came with lots of improvements and aggravation.

The most important improvement is dropping the `smarttabs` option therewith the absence of the annoying errors regarding »mixed tabs and spaces« within doc block comments like the following:
````javascript
/**
 * The jQuery object or a jQuery set containing on or more DOM elements.
 * @typedef {Object} jQuery
 */
````
There for local and latest JSHint is showing no errors while the JSHint report of plato is showing me errors regarding those »mixed tabs and spaces«.

I updated my local plato installation to JSHint 2.5.1 and it’s working like a charm :kissing_heart: