Hopefully we don't need the *latest* Node.js releases for the unit-tests to work on Travis, and by using "long-term support" releases instead we should be able to avoid these types of sudden failures in the future as well.
Letting Travis update npm packages can lead to unexpected and completely unrelated failures for any PR and/or merge, see e.g. 11719, if there's ever backwards *incompatible* changes when a package is updated.
The *exact* package versions are specified in `package-lock.json`, and they should thus be used when running tests. Note that the bots won't update npm packages, and neither should Travis as far as I'm concerned.
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.
