del 2.x has no callback function

[KiT106]

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

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

So, clean function should upgrade like:

// 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);
}

[zakhttp]

del 2.x supports promises, below is my implementation:

function clean(path, done) {
    log('Cleaning assets: ' + $.util.colors.blue(path));
    del(path)
        .then(function sucess() {
            done();
        })
        .catch(function failure(exception) {
            log('Cleaning assets exception: ' + exception);
        });
}