[ruby/psych] psych depends stringio only CRuby

https://github.com/ruby/psych/commit/e7bbf26cb2

ext/psych/psych.gemspec

@@ -63,7 +63,7 @@ DESCRIPTION
     s.add_dependency 'jar-dependencies', '>= 0.1.7'
   else
     s.extensions = ["ext/psych/extconf.rb"]
+    s.add_dependency 'stringio'
   end
 
-  s.add_dependency 'stringio'
 end