[ruby/irb] New IRB needs Ruby 2.5 or later

https://github.com/ruby/irb/commit/4be3158358

lib/irb/irb.gemspec

@@ -75,7 +75,7 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.required_ruby_version = Gem::Requirement.new(">= 2.4")
+  spec.required_ruby_version = Gem::Requirement.new(">= 2.5")
 
   spec.add_dependency "reline", ">= 0.0.1"
   spec.add_development_dependency "bundler"