Bump acorn from 5.7.1 to 5.7.4

Bumps [acorn](https://github.com/acornjs/acorn) from 5.7.1 to 5.7.4.
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](acornjs/acorn@5.7.1...5.7.4)

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

Author: dependabot[bot]