Upgrade to Gulp 4

This required the following changes in the Gulpfile:

- Defining a series of tasks is no longer done with arrays, but with the
  `gulp.series` function. The `web` target is refactored to use a
  smaller number of tasks to prevent tasks from running multiple times.
- Getting all tasks must now be done through the task registry.
- Tasks that don't return anything must call `done` upon completion.

Moreover, this upgrade allows us to use the latest Node.js on Travis CI
again.

examples/browserify/gulpfile.js

@@ -33,4 +33,4 @@ gulp.task('build-worker', function() {
     .pipe(gulp.dest(OUTPUT_PATH));
 });
 
-gulp.task('build', ['build-bundle', 'build-worker']);
+gulp.task('build', gulp.series('build-bundle', 'build-worker'));