Use links instead of depends_on in Docker compose files (#97)

docker-compose.prod.yml

@@ -16,7 +16,7 @@ services:
       - '8000'
     volumes:
       - www:/backend/www
-    depends_on:
+    links:
       - db
     env_file:
       - .env

docker-compose.yml

@@ -8,7 +8,7 @@ services:
     build: ./backend
     expose:
       - '8000'
-    depends_on:
+    links:
       - db
     volumes:
       - ./backend:/backend