Switch to DockerHub for Elasticsearch image

We were being rate-limited by docker.elastic.co:

```
Unavailable: Back-end server is at capacity (executor_docker.go:188:0s)
elasticsearch : 5.6.12
```

.gitlab-ci.yml

@@ -24,7 +24,7 @@ variables:
   FLAKY_RSPEC_SUITE_REPORT_PATH: rspec_flaky/report-suite.json
   BUILD_ASSETS_IMAGE: "false"
   ES_JAVA_OPTS: "-Xms256m -Xmx256m"
-  ELASTIC_URL: "http://elastic:changeme@docker.elastic.co-elasticsearch-elasticsearch:9200"
+  ELASTIC_URL: "http://elastic:changeme@elasticsearch:9200"
 
 after_script:
   - date

.gitlab/ci/global.gitlab-ci.yml

@@ -213,7 +213,7 @@
     - name: postgres:9.6
       command: ["postgres", "-c", "fsync=off", "-c", "synchronous_commit=off", "-c", "full_page_writes=off"]
     - name: redis:alpine
-    - name: docker.elastic.co/elasticsearch/elasticsearch:5.6.12
+    - name: elasticsearch:5.6.12
 
 .use-pg10-ee:
   image: "registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.6.3-golang-1.12-git-2.24-lfs-2.9-chrome-73.0-node-12.x-yarn-1.16-postgresql-10-graphicsmagick-1.3.33"
@@ -221,7 +221,7 @@
     - name: postgres:10.9
       command: ["postgres", "-c", "fsync=off", "-c", "synchronous_commit=off", "-c", "full_page_writes=off"]
     - name: redis:alpine
-    - name: docker.elastic.co/elasticsearch/elasticsearch:5.6.12
+    - name: elasticsearch:5.6.12
 
 .only-ee:
   only: