Add support for PG15 in travis

Author: Maxim Orlov

.travis.yml

@@ -20,6 +20,9 @@ notifications:
         on_failure: always
 
 env:
+    - PG_VERSION=15 LEVEL=nightmare
+    - PG_VERSION=15 LEVEL=hardcore
+    - PG_VERSION=15
     - PG_VERSION=14 LEVEL=nightmare
     - PG_VERSION=14 LEVEL=hardcore
     - PG_VERSION=14
@@ -41,3 +44,4 @@ matrix:
         - env: PG_VERSION=12 LEVEL=nightmare
         - env: PG_VERSION=13 LEVEL=nightmare
         - env: PG_VERSION=14 LEVEL=nightmare
+        - env: PG_VERSION=15 LEVEL=nightmare