Update rubocop requirement to ~> 0.54

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)

gc_ruboconfig.gemspec

@@ -8,6 +8,6 @@ Gem::Specification.new do |spec|
   spec.license       = "MIT"
 
   spec.files         = "rubocop.yml"
-  spec.add_dependency "rubocop", "~> 0.53.0"
+  spec.add_dependency "rubocop", "~> 0.54.0"
   spec.add_dependency "rubocop-rspec", "~> 1.24"
 end