Add JRuby 9.3.4.0 to Dockerfile and use it for 9.3.x testing.

kokoro/linux/dockerfile/test/ruby/Dockerfile

@@ -38,6 +38,7 @@ RUN /bin/bash -l -c "rvm install 3.0.0"
 RUN /bin/bash -l -c "rvm install 3.1.0"
 RUN /bin/bash -l -c "rvm install jruby-9.2.20.1"
 RUN /bin/bash -l -c "rvm install jruby-9.3.3.0"
+RUN /bin/bash -l -c "rvm install jruby-9.3.4.0"
 
 RUN /bin/bash -l -c "echo 'gem: --no-ri --no-rdoc' > ~/.gemrc"
 RUN /bin/bash -l -c "echo 'export PATH=/usr/local/rvm/bin:$PATH' >> ~/.bashrc"

tests.sh

@@ -444,7 +444,7 @@ build_jruby92() {
 build_jruby93() {
   internal_build_cpp                # For conformance tests.
   internal_build_java jdk8 && cd .. # For Maven protobuf jar with local changes
-  cd ruby && bash travis-test.sh jruby-9.3.3.0 && cd ..
+  cd ruby && bash travis-test.sh jruby-9.3.4.0 && cd ..
 }
 
 build_javascript() {