Update jshint from 2.4.0 to 2.4.3.

In jshint 2.4.0, there is a conflict between the `esnext` and `undef` options:

```javascript
// jshint undef: true, esnext: true
var doubler = n => n * 2;
```

In this example, `doubler` is a valid JS function, equivalent to `function(n) {return n * 2;}`, but the `undef` validation emits an error. This conflict is resolved in jshint 2.4.3.

Based on [the most recent jshint update pull request](https://github.com/gruntjs/grunt-contrib-jshint/pull/128), I wasn't sure how to structure this one. I'd welcome your advice!