fix: limit the rubocop version to avoid breaking changes

Author: Sam Harrison

Diff for: ruby_http_client.gemspec

@@ -18,6 +18,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'codecov'
   spec.add_development_dependency 'minitest'
   spec.add_development_dependency 'rake'
-  spec.add_development_dependency 'rubocop'
+  spec.add_development_dependency 'rubocop', '~> 0.88.0'
   spec.add_development_dependency 'simplecov'
 end