Merge branch 'sh-bump-qa-image-ruby-2.5' into 'master'

Upgrade GitLab QA image to Ruby 2.5

See merge request gitlab-org/gitlab-ce!24091

qa/Dockerfile

-FROM ruby:2.4-stretch
+FROM ruby:2.5-stretch
 LABEL maintainer "Grzegorz Bizon <grzegorz@gitlab.com>"
 ENV DEBIAN_FRONTEND noninteractive