Update laravel-mix to the latest version 🚀 (delaford#18)

## The devDependency [laravel-mix](https://github.com/JeffreyWay/laravel-mix) was updated from `3.0.0` to `4.0.0`.
This version is **not covered** by your **current version range**.

If you don’t accept this pull request, your project will work just like it did before. However, you might be missing out on a bunch of new features, fixes and/or performance improvements from the dependency update.

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[Find out more about this release](https://github.com/JeffreyWay/laravel-mix).

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