Convert "gulp" into a peer-dependency and update for ^4.0.0
Convert "gulp" into a peer-dependency and update for ^4.0.0

This package doesn't really care what kind of gulp the consumer decides to use, and so we definitely
don't want to force them down to 3.9.1. For our own build environment 4.0.0 is fine, and this package
will be (from now) tested against 4.0.0.

Collaborne/gulp-cordova-build-utils,Collaborne/gulp-cordova-build-utils