[blavenie]

Upgrade gulp@4.0.2, to fix build error, on Node 12+
See #4114 (comment)

[blavenie]

Now it seems to work fine.

This patch allow:

• to use Gulp 4 syntax in <IONIC_PROJECT>/gulpfile.js
• Failed if gulp V3 still use, but display an explicit error (with a link to the gulp v4 migration guide);
• Wait end of tasks (e.g. ionic serve will wait end of ionic:serve:before - fix an existing issue);

[imhoffd]

@blavenie You can run npm run lint:fix to fix all the lint issues reported in CI.

[imhoffd]

Thanks @blavenie! This will be released shortly.