Replace deprecated dependency gulp-util

[demurgos]

gulp-util has been recently deprecated. Continuing to use this dependency may prevent the use of your library with the latest release of Gulp 4 so it is important to replace gulp-util.

The README.md lists alternatives for all the components so a simple replacement should be enough.

Your package is one of the most popular packages still relying on gulp-util, it would be good to publish a fixed version to npm as soon as possible.

See:

• https://medium.com/gulpjs/gulp-util-ca3b1f9f9ac5
• Ecosystem migration gulpjs/gulp-util#143