[Security] Bump acorn from 6.4.0 to 6.4.1

Bumps [acorn](https://github.com/acornjs/acorn) from 6.4.0 to 6.4.1. **This update includes security fixes.**
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](acornjs/acorn@6.4.0...6.4.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>