gitlab-ci: use include:template to bring in AutoDevOps

Now we're on GitLab 11.7, we can use include:template: to bring in the
default AutoDevOps template.

.gitlab-ci.yml

@@ -5,9 +5,7 @@
 
 include:
   # Bring in the AutoDevOps template from GitLab.
-  #
-  # TODO: when we ship GitLab 11.7, replace this with include:template:
-  - 'https://gitlab.com/gitlab-org/gitlab-ee/raw/master/lib/gitlab/ci/templates/Auto-DevOps.gitlab-ci.yml'
+  - template: Auto-DevOps.gitlab-ci.yml
 
 variables:
   # Disable bits of the AutoDevOps pipeline which we don't use.