Add PostgreSQL 12 tests

Author: brianhlin

.travis.yml

@@ -3,6 +3,15 @@ language: go
 
 matrix:
   include:
+    - name: "Postgres 12"
+      env:
+        - PGDB=12
+      addons:
+        postgresql: "12"
+        apt:
+          packages:
+            - postgresql-12
+            - postgresql-client-12
     - name: "Postgres 11"
       env:
         - PGDB=11