Bump acorn from 6.3.0 to 6.4.1 (#18)

Bumps [acorn](https://github.com/acornjs/acorn) from 6.3.0 to 6.4.1.
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](acornjs/acorn@6.3.0...6.4.1)

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