del 2.x has no callback function

``` javascript
// del 1.x
del(patterns, [options], callback)

// del 2.x
del(patterns, [options])
```

So, `clean` function should upgrade like:

``` javascript
// with del 1.x
function clean(path, done) {
log('Cleaning: ' + $.util.colors.blue(path));
del(path, done);
}

// with del 2.x
function clean(path) {
return del(config.distribution.scripts);
}
```