Bump GitLab CI test image to use git v2.16

git 2.15.0 has some significant performance improvements when running `git
fetch` on a repository that has lots of references. See
https://gitlab.com/gitlab-com/migration/issues/216#note_60672987 for more
details.

.gitlab-ci.yml

-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"
+image: "dev.gitlab.org:5005/gitlab/gitlab-build-images:ruby-2.3.6-golang-1.9-git-2.16-chrome-63.0-node-8.x-yarn-1.2-postgresql-9.6"
 
 .dedicated-runner: &dedicated-runner
   retry: 1