Upgrade PostgreSQL versions in CI

* 9.6.11 -> 9.6.14
* 10.7 -> 10.9

This is done in preparation for upgrading PostgreSQL in Omnibus:
https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/3492

.gitlab/ci/frontend.gitlab-ci.yml

@@ -8,7 +8,7 @@
 .use-pg: &use-pg
   services:
-    - name: postgres:9.6.11
+    - name: postgres:9.6.14
       command: ["postgres", "-c", "fsync=off", "-c", "synchronous_commit=off", "-c", "full_page_writes=off"]
     - name: redis:alpine

.gitlab/ci/rails.gitlab-ci.yml

 .use-pg: &use-pg
   services:
-    - name: postgres:9.6.11
+    - name: postgres:9.6.14
       command: ["postgres", "-c", "fsync=off", "-c", "synchronous_commit=off", "-c", "full_page_writes=off"]
     - name: redis:alpine
 .use-pg-10: &use-pg-10
   services:
-    - name: postgres:10.7
+    - name: postgres:10.9
       command: ["postgres", "-c", "fsync=off", "-c", "synchronous_commit=off", "-c", "full_page_writes=off"]
     - name: redis:alpine