Require Ruby 2.4 or later

Match the gem requirement to the oldest Ruby version covered by
the test matrix.

Author: nobu

pstore.gemspec

@@ -17,6 +17,7 @@ Gem::Specification.new do |spec|
   spec.description   = spec.summary
   spec.homepage      = "https://github.com/ruby/pstore"
   spec.licenses      = ["Ruby", "BSD-2-Clause"]
+  spec.required_ruby_version = ">= 2.4"
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = "https://github.com/ruby/pstore"