Bump acorn from 6.4.0 to 6.4.1#2

Bumps [acorn](https://github.com/acornjs/acorn) from 6.4.0 to 6.4.1. <details> <summary>Commits</summary> <ul> <li><a href="https://github.com/acornjs/acorn/commit/9a2e9b6678e243d66846b91179d650d28453e70c"><code>9a2e9b6</code></a> Mark version 6.4.1</li> <li><a href="https://github.com/acornjs/acorn/commit/90a9548ea0ce351b54f956e2c4ed27cca9631284"><code>90a9548</code></a> More rigorously check surrogate pairs in regexp validator</li> <li>See full diff in <a href="https://github.com/acornjs/acorn/compare/6.4.0...6.4.1">compare view</a></li> </ul> </details> <br /> [](https://help.github.com/articles/configuring-automated-security-fixes) Dependabot will resolve any conflicts with this PR as long as you don't alter it yourself. You can also trigger a rebase manually by commenting `@dependabot rebase`. [//]: # (dependabot-automerge-start) [//]: # (dependabot-automerge-end) --- <details> <summary>Dependabot commands and options</summary> <br /> You can trigger Dependabot actions by commenting on this PR: - `@dependabot rebase` will rebase this PR - `@dependabot recreate` will recreate this PR, overwriting any edits that have been made to it - `@dependabot merge` will merge this PR after your CI passes on it - `@dependabot squash and merge` will squash and merge this PR after your CI passes on it - `@dependabot cancel merge` will cancel a previously requested merge and block automerging - `@dependabot reopen` will reopen this PR if it is closed - `@dependabot close` will close this PR and stop Dependabot recreating it. You can achieve the same result by closing it manually - `@dependabot ignore this major version` will close this PR and stop Dependabot creating any more for this major version (unless you reopen the PR or upgrade to it yourself) - `@dependabot ignore this minor version` will close this PR and stop Dependabot creating any more for this minor version (unless you reopen the PR or upgrade to it yourself) - `@dependabot ignore this dependency` will close this PR and stop Dependabot creating any more for this dependency (unless you reopen the PR or upgrade to it yourself) - `@dependabot use these labels` will set the current labels as the default for future PRs for this repo and language - `@dependabot use these reviewers` will set the current reviewers as the default for future PRs for this repo and language - `@dependabot use these assignees` will set the current assignees as the default for future PRs for this repo and language - `@dependabot use this milestone` will set the current milestone as the default for future PRs for this repo and language You can disable automated security fix PRs for this repo from the [Security Alerts page](https://github.com/jakeboone02/format-quantity/network/alerts). </details>