Bump acorn from 6.1.1 to 6.4.2

Bumps [acorn](https://github.com/acornjs/acorn) from 6.1.1 to 6.4.2.
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](acornjs/acorn@6.1.1...6.4.2)

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

Author: dependabot[bot]