Add reason to keep postgresql 9.2 for CI

.gitlab-ci.yml

@@ -61,6 +61,9 @@ stages:
 .use-pg: &use-pg
   services:
+    # As of Jan 2018, we don't have a strong reason to upgrade to 9.6 for CI yet,
+    # so using the least common denominator ensures backwards compatibility
+    # (as many users are still using 9.2).
     - postgres:9.2
     - redis:alpine

changelogs/unreleased/41749-postgres-9-6-for-ci-tests.yml

+---
+title: Add reason to keep postgresql 9.2 for CI
+merge_request: 16277
+author: Takuya Noguchi
+type: other