Ensure rubocop does not install on Ruby 2.4.
Ensure rubocop does not install on Ruby 2.4.

rubocop 0.23 depends on a version of json with native extensions
that fail to compile on Ruby 2.4. The platform directive does not
prevent it from being installed on Ruby 2.4 for some reason.
rspec/rspec-expectations,rspec/rspec-expectations