Update rubocop requirement from ~> 0.63.0 to ~> 0.64.0

Updates the requirements on [rubocop](https://github.com/rubocop-hq/rubocop) to permit the latest version.
- [Release notes](https://github.com/rubocop-hq/rubocop/releases)
- [Changelog](https://github.com/rubocop-hq/rubocop/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rubocop-hq/rubocop/commits/v0.64.0)

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

colorls.gemspec

@@ -58,7 +58,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'ronn', '~> 0'
   spec.add_development_dependency 'rspec', '~> 3.7'
   spec.add_development_dependency 'rspec-its', '~> 1.2'
-  spec.add_development_dependency 'rubocop', '~> 0.63.0'
+  spec.add_development_dependency 'rubocop', '~> 0.64.0'
   spec.add_development_dependency 'rubocop-rspec', '~> 1.27'
   spec.add_development_dependency 'rubygems-tasks', '~> 0'
   spec.add_development_dependency 'simplecov', '~> 0.16.1'