Add release stage to push new gems on tag

Enables CI/CD on the GitLab QA project.

On tagging, let GitLab CI publish a new gem automatically, rather than a maintainer gem pushing manually.

The push command will use ~/.gem/credentials to authenticate to a server, but you can use the RubyGems environment variable GEM_HOST_API_KEY to set the api key to authenticate.

• create gitlab-qa rubygems account
• grant gitlab-qa access to gitlab-qa rubygem
• add GEM_HOST_API_KEY to CI/CD settings