Add building/testing PG14 and PG15beta1 in TravisCI

Author: maksm90

.travis.yml

@@ -29,3 +29,7 @@ env:
     - 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=15beta1  CHECK_CODE=clang
+    - PG_VERSION=15beta1  CHECK_CODE=false