Use :latest image tag in default Ruby CI template

Changelog: changed

lib/gitlab/ci/templates/Ruby.gitlab-ci.yml

 # Official language image. Look for the different tagged releases at:
 # https://hub.docker.com/r/library/ruby/tags/
-image: "ruby:2.5"
+image: ruby:latest
 
 # Pick zero or more services to be used on all builds.
 # Only needed when using a docker container to run your tests in.