Add more tests and allow failures for 9.6 for now

Author: ololobus

.travis.yml

@@ -23,5 +23,11 @@ env:
     - PG_VERSION=10  CHECK_CODE=clang
     - PG_VERSION=10  CHECK_CODE=cppcheck
     - PG_VERSION=10  CHECK_CODE=false
+    - PG_VERSION=11  CHECK_CODE=clang
     - PG_VERSION=11  CHECK_CODE=false
+    - PG_VERSION=12  CHECK_CODE=clang
     - PG_VERSION=12  CHECK_CODE=false
+
+jobs:
+    allow_failures:
+        - env: PG_VERSION=9.6 CHECK_CODE=false