Update rubocop requirement from ~> 1.63.1 to ~> 1.64.0

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@v1.63.1...v1.64.0)

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

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

oxidized.gemspec

@@ -38,7 +38,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'mocha',               '~> 2.1'
   s.add_development_dependency 'pry',                 '~> 0.14.2'
   s.add_development_dependency 'rake',                '~> 13.0'
-  s.add_development_dependency 'rubocop',             '~> 1.63.1'
+  s.add_development_dependency 'rubocop',             '~> 1.64.0'
   s.add_development_dependency 'rubocop-minitest',    '~> 0.35.0'
   s.add_development_dependency 'rubocop-rake',        '~> 0.6.0'
   s.add_development_dependency 'simplecov',           '~> 0.22.0'