⬆️ Bump acorn from 6.3.0 to 6.4.2

Bumps [acorn](https://github.com/acornjs/acorn) from 6.3.0 to 6.4.2.
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](acornjs/acorn@6.3.0...6.4.2)

---
updated-dependencies:
- dependency-name: acorn
  dependency-type: indirect
...

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