add pgsql ci job with stable15

.travis.yml

@@ -17,6 +17,9 @@ matrix:
           env: DB=sqlite
         - php: 7.2
           env: DB=mysql
+        - php: 7.2
+          env: DB=pgsql
+          services: postgresql
         - php: 7.2
           env: DB=pgsql CORE_BRANCH=master
           services: postgresql