Bug 26228: Update gulpfile to work with Node.js v12

Bug 26228: Update gulpfile to work with Node.js v12

This patch makes the gulpfile work with gulp v4 on Node v12. To test it,
make sure your dev env has Node.js v12.

It also replaces the use of 'gulp-util' (deprecated) by 'minimist' as
recommended on their site.

On both KTD and KohaDevBox edit your sources.list so the node line
points to 'node_12.x' instead of 'node_8.x'.

Once that's done:
1. On your clone run:
   $ yarn install
   $ npm install -E
2. Build the CSS:
   $ yarn build
   $ yarn build --view opac
=> SUCCESS: Things build correctly
3. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>