Updated test script to support actual pg versions for testing.

Author: Vitaly Davydov

.travis.yml

@@ -2,13 +2,12 @@
 ---
 # versions to run on
 env:
-  - PG_SUPPORTED_VERSIONS=9.4 # introduces psprintf
-  - PG_SUPPORTED_VERSIONS=9.5
-  - PG_SUPPORTED_VERSIONS=9.6
   - PG_SUPPORTED_VERSIONS=10
   - PG_SUPPORTED_VERSIONS=11
   - PG_SUPPORTED_VERSIONS=12
   - PG_SUPPORTED_VERSIONS=13
+  - PG_SUPPORTED_VERSIONS=14
+  - PG_SUPPORTED_VERSIONS=15
 
 language: C
 dist: bionic