Add PostgreSQL 11 into Travis tests

Author: za-arthur

.travis.yml

@@ -18,12 +18,13 @@ notifications:
         on_failure: always
 
 env:
-    - PG_VERSION=10 LEVEL=nightmare
-    - PG_VERSION=10 LEVEL=hardcore
+    - PG_VERSION=11 LEVEL=nightmare
+    - PG_VERSION=11 LEVEL=hardcore
+    - PG_VERSION=11
     - PG_VERSION=10
     - PG_VERSION=9.6
     - PG_VERSION=9.5
 
 matrix:
     allow_failures:
-        - env: PG_VERSION=10 LEVEL=nightmare
+        - env: PG_VERSION=11 LEVEL=nightmare