[Security] Bump acorn from 6.3.0 to 6.4.2 (#377)

Bumps [acorn](https://github.com/acornjs/acorn) from 6.3.0 to 6.4.2. **This update includes security fixes.**
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](acornjs/acorn@6.3.0...6.4.2)

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]