Fix gulp debug target

[chrisekelley]

The clean target is not finished before the version target needs to run. A couple tings need to happen:
Upgrade to gulp 4 - some asyn issues are resolved in the 4 release
Then add a callback to the clean target - wait, there is already a done cb. So maybe we just need to upgrade to gulp 4, which is not yet but will soon be released.

gulpjs/gulp#899
http://stackoverflow.com/questions/22824546/how-to-run-gulp-tasks-synchronously-one-after-the-other
https://github.com/gulpjs/gulp/blob/master/docs/API.md#async-task-support