bring back ruby 2.4 since that broke rubocop builds because of some d…

…ependency foobar

parallel.gemspec

@@ -15,5 +15,5 @@ Gem::Specification.new name, Parallel::VERSION do |s|
   }
   s.files = `git ls-files lib MIT-LICENSE.txt`.split("\n")
   s.license = "MIT"
-  s.required_ruby_version = '>= 2.5'
+  s.required_ruby_version = '>= 2.4'
 end