[ruby/stringio] Move Java version to Java directory

https://github.com/ruby/stringio/commit/3f90a0d619

ext/stringio/stringio.gemspec

@@ -21,7 +21,8 @@ Gem::Specification.new do |s|
   s.files = ["README.md"]
   jruby = true if Gem::Platform.new('java') =~ s.platform or RUBY_ENGINE == 'jruby'
   if jruby
-    s.files += ["lib/stringio.rb", "lib/stringio.jar"]
+    s.require_paths = "lib/java"
+    s.files += ["lib/java/stringio.rb", "lib/java/stringio.jar"]
     s.platform = "java"
   else
     s.extensions = ["ext/stringio/extconf.rb"]