Update rubocop requirement to ~> 0.54.0

Updates the requirements on [rubocop](https://github.com/bbatsov/rubocop) to permit the latest version.
- [Release notes](https://github.com/bbatsov/rubocop/releases)
- [Changelog](https://github.com/bbatsov/rubocop/blob/master/CHANGELOG.md)
- [Commits](https://github.com/bbatsov/rubocop/commits/v0.54.0)

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

dependabot-core.gemspec

@@ -35,6 +35,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "rake"
   spec.add_development_dependency "rspec", "~> 3.7.0"
   spec.add_development_dependency "rspec-its", "~> 1.2.0"
-  spec.add_development_dependency "rubocop", "~> 0.53.0"
+  spec.add_development_dependency "rubocop", "~> 0.54.0"
   spec.add_development_dependency "webmock", "~> 3.3.0"
 end