Add depends_on to ALLOWED_KEYS

This ensures and already existing `depends_on` is not deleted when the service on which it is defined also employs `extends`.

Signed-off-by: Ryan Taylor Long <[email protected]>

compose/config/config.py

@@ -84,6 +84,7 @@ DOCKER_CONFIG_KEYS = [
 ALLOWED_KEYS = DOCKER_CONFIG_KEYS + [
     'build',
     'container_name',
+    'depends_on',
     'dockerfile',
     'expose',
     'external_links',