Bump acorn from 6.1.1 to 6.4.1

Bumps [acorn](https://github.com/acornjs/acorn) from 6.1.1 to 6.4.1.
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](acornjs/acorn@6.1.1...6.4.1)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]