Add auto_approve to hello-world exercise (#415)

This adds the auto_approve property to the hello-world 
exercise as part of preparing tracks for the v2 launch.

The current code on the server will automatically approve
hello-world exercises anyway, but that is a temporary fix
and will be removed in the future.

See #412

Author: jackhughesweb

config.json

@@ -8,6 +8,7 @@
       "slug": "hello-world",
       "uuid": "9ce0f408-6d7b-4466-a390-75aeaf9492f2",
       "core": true,
+      "auto_approve": true,
       "unlocked_by": null,
       "difficulty": 1,
       "topics": [