Bump cross-spawn, cross-env and laravel-mix

Bumps [cross-spawn](https://github.com/moxystudio/node-cross-spawn) to 7.0.5 and updates ancestor dependencies [cross-spawn](https://github.com/moxystudio/node-cross-spawn), [cross-env](https://github.com/kentcdodds/cross-env) and [laravel-mix](https://github.com/JeffreyWay/laravel-mix). These dependencies need to be updated together.


Updates `cross-spawn` from 6.0.5 to 7.0.5
- [Changelog](https://github.com/moxystudio/node-cross-spawn/blob/master/CHANGELOG.md)
- [Commits](moxystudio/node-cross-spawn@v6.0.5...v7.0.5)

Updates `cross-env` from 5.2.1 to 7.0.3
- [Release notes](https://github.com/kentcdodds/cross-env/releases)
- [Changelog](https://github.com/kentcdodds/cross-env/blob/master/CHANGELOG.md)
- [Commits](kentcdodds/cross-env@v5.2.1...v7.0.3)

Updates `laravel-mix` from 4.1.4 to 6.0.49
- [Release notes](https://github.com/JeffreyWay/laravel-mix/releases)
- [Changelog](https://github.com/laravel-mix/laravel-mix/blob/master/CHANGELOG.md)
- [Upgrade guide](https://github.com/laravel-mix/laravel-mix/blob/master/UPGRADE.md)
- [Commits](https://github.com/JeffreyWay/laravel-mix/commits)

---
updated-dependencies:
- dependency-name: cross-spawn
  dependency-type: indirect
- dependency-name: cross-env
  dependency-type: direct:development
- dependency-name: laravel-mix
  dependency-type: direct:development
...

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