change links to depends_on

This causes containers to be started in dependency order and this
was the actual intention. Links are implicit by default.

docker-compose.yml

@@ -2,7 +2,7 @@ version: '3'
 services:
   web:
     build: web
-    links:
+    depends_on:
     - backend
     environment:
       ICEBOX_API_HOST: backend
@@ -12,11 +12,11 @@ services:
     build: alias
     environment:
     - ICEBOX_DB_URL=postgresql://icebox:clubmate42@database/icebox
-    links:
+    depends_on:
     - database
   backend:
     build: backend
-    links:
+    depends_on:
     - database
     environment:
       ICEBOX_DB_HOST: database
@@ -30,7 +30,7 @@ services:
     - "80:80"
     - "8081:8081"
     - "8085:8085"
-    links:
+    depends_on:
     - web
     - alias
     - backend