fix(ruby): update rubygems

Update rubygems for vulnerabilities listed here:
# https://www.ruby-lang.org/en/news/2017/08/29/multiple-vulnerabilities-in-rubygems/

Author: bethesque

Diff for: Dockerfile

@@ -16,6 +16,10 @@ ADD pact_broker/Gemfile $APP_HOME/
 ADD pact_broker/Gemfile.lock $APP_HOME/
 RUN chown -R app:app $APP_HOME
 
+# Update system gems for:
+# https://www.ruby-lang.org/en/news/2017/08/29/multiple-vulnerabilities-in-rubygems/
+RUN gem update --system
+RUN gem install bundler
 RUN su app -c "cd $APP_HOME && bundle install --deployment --without='development test'"
 ADD pact_broker/ $APP_HOME/
 RUN chown -R app:app $APP_HOME