Add 12 to travis.

Author: arssher

Diff for: .travis.yml

@@ -18,6 +18,8 @@ notifications:
         on_failure: always
 
 env:
+    - PG_VERSION=12 LEVEL=hardcore
+    - PG_VERSION=12
     - PG_VERSION=11 LEVEL=hardcore
     - PG_VERSION=11
     - PG_VERSION=10 LEVEL=hardcore