Because the "return promise if no callback" from async combines really
well with the implicit return from CoffeeScript.

I discovered this bug after the previous commit. See
https://github.com/paulmillr/chokidar/issues/612#issuecomment-378026034

This solves the issue with the fs change event test timing out most of
the time. It also fixes a real bug I didn't realize I had, yet.

Does not solve the timeout problem, but does improve maintainability.

TODO:
- one of the tests times out most of the time, find out why
- use utilities from the async package

This also reverts commit 76c140c4.