Update rubocop requirement from 1.54.0 to 1.55.1 (#149)

Updates the requirements on [rubocop](https://github.com/rubocop/rubocop) to permit the latest version.
- [Release notes](https://github.com/rubocop/rubocop/releases)
- [Changelog](https://github.com/rubocop/rubocop/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop@v1.54.0...v1.55.1)

---
updated-dependencies:
- dependency-name: rubocop
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile

@@ -4,7 +4,7 @@ gemspec
 gem "minitest", "~> 5.11"
 gem "minitest-reporters", "~> 1.3"
 gem "rake", "~> 13.0"
-gem "rubocop", "1.54.0"
+gem "rubocop", "1.55.1"
 gem "rubocop-minitest", "0.31.0"
 gem "rubocop-packaging", "0.5.2"
 gem "rubocop-performance", "1.18.0"