Flatten authors and contributors array

The authors and contributors of exercises are stored in an array in the exercises' `.meta/config.json` files.
We used to have separate fields for the GitHub- and Exercism username of the user.
As we're only using the GitHub username, we've flatten the `authors` and `contributors` array to an array of strings representing the GitHub usernames.

See https://github.com/exercism/docs/pull/90/files

Author: ErikSchierboom

exercises/concept/lucians-luscious-lasagna/.meta/config.json

@@ -1,14 +1,8 @@
 {
   "blurb": "TODO: add blurb for lucians-luscious-lasagna exercise",
   "authors": [
-    {
-      "github_username": "mpizenberg",
-      "exercism_username": "mpizenberg"
-    },
-    {
-      "github_username": "ceddlyburge",
-      "exercism_username": "ceddlyburge"
-    }
+    "mpizenberg",
+    "ceddlyburge"
   ],
   "forked_from": [
     "fsharp/lucians-luscious-lasagna"