Set up Kokoro to build JRuby gems

kokoro/release/ruby/linux/ruby/ruby_build.sh

@@ -11,7 +11,14 @@ fi
 
 umask 0022
 pushd ruby
+
 bundle install && bundle exec rake gem:native
 ls pkg
 mv pkg/* $ARTIFACT_DIR
+
+# JRuby
+rvm use jruby
+rake build && gem build google-protobuf.gemspec
+mv google-protobuf-*-java.gem $ARTIFACT_DIR
+
 popd