use callback to allow correct task dep definition

Update this to prevent copy & paste "errors" of this demo

Extract from the gulp api doc:

Note: Are your tasks running before the dependencies are complete? Make sure your dependency tasks are correctly using the async run hints: take in a callback or return a promise or event stream.

So your critical package doesn't return a promise ( * but as it looks like it's easy to extend https://github.com/addyosmani/critical/blob/master/index.js )