Bump webpacker from 4.0.5 to 4.0.6 (#10905)

Bumps [webpacker](https://github.com/rails/webpacker) from 4.0.5 to 4.0.6.
- [Release notes](https://github.com/rails/webpacker/releases)
- [Changelog](https://github.com/rails/webpacker/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rails/webpacker/compare/v4.0.5...v4.0.6)