Add more postgres versions to tests

Author: ildus

.travis.yml

@@ -21,6 +21,8 @@ env:
     - PYTHON_VERSION=2 PG_VERSION=10
     - PYTHON_VERSION=2 PG_VERSION=9.6
     - PYTHON_VERSION=2 PG_VERSION=9.5
+    - PYTHON_VERSION=2 PG_VERSION=9.4
     - PYTHON_VERSION=3 PG_VERSION=10
     - PYTHON_VERSION=3 PG_VERSION=9.6
     - PYTHON_VERSION=3 PG_VERSION=9.5
+    - PYTHON_VERSION=3 PG_VERSION=9.4