Update the license for the default gems to dual licenses

Merged ruby/ruby@8fb02b7

Author: hsbt

stringio.gemspec

@@ -22,7 +22,7 @@ Gem::Specification.new do |s|
   s.extensions = ["ext/stringio/extconf.rb"]
   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.licenses = ["Ruby", "BSD-2-Clause"]
   s.required_ruby_version = ">= 2.5"
   s.summary = "Pseudo IO on String"