Dropped older ruby versions

Author: nobu

stringio.gemspec

@@ -23,7 +23,7 @@ Gem::Specification.new do |s|
   s.files = ["README.md", "ext/stringio/extconf.rb", "ext/stringio/stringio.c"]
   s.homepage = "https://github.com/ruby/stringio"
   s.licenses = ["BSD-2-Clause"]
-  s.required_ruby_version = Gem::Requirement.new(">= 2.2")
+  s.required_ruby_version = ">= 2.4"
   s.rubygems_version = "2.6.11"
   s.summary = "Pseudo IO on String"