Update Ruby version to 2.3.6

Signed-off-by: Rémy Coutable <remy@rymai.me>

.gitlab-ci.yml

-image: "dev.gitlab.org:5005/gitlab/gitlab-build-images:ruby-2.3.5-golang-1.8-git-2.14-chrome-63.0-node-8.x-yarn-1.2-postgresql-9.6"
+image: "dev.gitlab.org:5005/gitlab/gitlab-build-images:ruby-2.3.6-golang-1.9-git-2.14-chrome-63.0-node-8.x-yarn-1.2-postgresql-9.6"
 .dedicated-runner: &dedicated-runner
   retry: 1

.ruby-version

-2.3.5
+2.3.6

changelogs/unreleased/41268-bump-ruby-to-2-3-6.yml

+---
+title: Upgrade Ruby to 2.3.6 to include security patches
+merge_request: 16016
+author:
+type: security