Use postgresql-9.6-postgis-2.3 on Travis

Author: pka

.travis.yml

@@ -7,7 +7,10 @@ rust:
 services:
   - postgresql
 addons:
-  postgresql: "9.4"
+  postgresql: 9.6
+  apt:
+    packages:
+    - postgresql-9.6-postgis-2.3
 env:
   DBCONN=postgresql://postgres@127.0.0.1/testdb