Merge pull request #1056 from bmkiefer/reverse-string-behind-two-fer

reverse-string: change unlocked_by exercise to two-fer

config.json

       "topics": [
         "strings"
       ],
-      "unlocked_by": null,
+      "unlocked_by": "two-fer",
       "uuid": "2c8afeed-480e-41f3-ad58-590fa8f88029"
     },
     {