Bump acorn from 5.7.3 to 5.7.4 (#28)

Bumps [acorn](https://github.com/acornjs/acorn) from 5.7.3 to 5.7.4.
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](acornjs/acorn@5.7.3...5.7.4)

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

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