update service for gitlab-ci

(cherry picked from commit b1397b4d)

.gitlab-ci.yml

@@ -7,7 +7,7 @@ stages:
 # Only needed when using a docker container to run your tests in.
 # Check out: http://docs.gitlab.com/ce/ci/docker/using_docker_images.html#what-is-a-service
 services:
-  - docker:dind
+  - docker:18-dind
 
 variables:
   POSTGRES_DB: postgres