Update .drone.yml for Drone 0.5 (#2761)

.drone.yml

@@ -1,6 +1,6 @@
 # The scripts for building the base images can be found in scripts/drone-images
 
-build:
+pipeline:
   flake8:
     image: python:3.5
     commands:
@@ -19,7 +19,7 @@ build:
       - npm run lint
       - npm run test:unit
   scss-lint:
-    image: wagtail-scss-lint
+    image: torchbox/scss-lint
     commands:
       - scss-lint
   unittest: