Upgrade GitLab QA image to Ruby 2.5

GitLab is now shipping with Ruby 2.5.3, so we should make the
version consistent for the QA image as well.

qa/Dockerfile

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