Use vanilla postgres image instead of our custom version

Author: bnaul

docker-compose/docker-compose.yml

@@ -8,8 +8,6 @@ web:
     - .:/home/cesium_app
 
 db:
-  image: cesium/postgres
+  image: postgres:9.5
   environment:
     - POSTGRES_USER=cesium
-    - POSTGRES_PASSWORD=
-    - POSTGRES_DB=cesium