Bump @rails/webpacker from 5.1.1 to 5.2.1 (#12)

Bumps [@rails/webpacker](https://github.com/rails/webpacker) from 5.1.1 to 5.2.1. - [Release notes](https://github.com/rails/webpacker/releases) - [Changelog](https://github.com/rails/webpacker/blob/v5.2.1/CHANGELOG.md) - [Commits](rails/webpacker@v5.1.1...v5.2.1) Signed-off-by: dependabot-preview[bot] <support@dependabot.com> Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]