Migrate to gulp v4

[Silic0nS0ldier]

Nice as it would be to stop using gulp inside entirely, its simply not worth the effort. Any projects (cough cough UserFrosting) that use this library will however be locked into v3.

In the name of making migration to the shinny new v4 as smooth as possible, we update this dependency to gulp v4 as part of a major update.
As this will be a drastic change, we'll also need to get unit testing fixed 😱. Who knows, maybe as part of the major update I can find a way to grip gulp out completely without doing a complete overhaul. That would be a neat feat.

[Silic0nS0ldier]

See #6