Add building/testing with PG14 and PG15beta3 in Travis CI.

Drop builds for PG9.6.

Author: maksm90

Diff for: .travis.yml

@@ -17,15 +17,16 @@ script:
     - docker-compose run tests
 
 env:
-    - PG_VERSION=9.6 CHECK_CODE=clang
-    - PG_VERSION=9.6 CHECK_CODE=cppcheck
-    - PG_VERSION=9.6 CHECK_CODE=false
-    - 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
-    - PG_VERSION=13  CHECK_CODE=clang
-    - PG_VERSION=13  CHECK_CODE=false
+    - 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
+    - PG_VERSION=13      CHECK_CODE=clang
+    - PG_VERSION=13      CHECK_CODE=false
+    - PG_VERSION=14      CHECK_CODE=clang
+    - PG_VERSION=14      CHECK_CODE=false
+    - PG_VERSION=15beta3 CHECK_CODE=clang
+    - PG_VERSION=15beta3 CHECK_CODE=false