move reverse-string exercise behind two-fer

config.json

       "uuid": "f77dc7e3-35a8-4300-a7c8-2c1765e9644d"
     },
     {
-      "core": false,
+      "core": true,
       "difficulty": 1,
-      "slug": "reverse-string",
+      "slug": "two-fer",
       "topics": [
+        "conditionals",
         "strings"
       ],
       "unlocked_by": null,
-      "uuid": "2c8afeed-480e-41f3-ad58-590fa8f88029"
+      "uuid": "74515d45-565b-4be2-96c4-77e58efa9257"
     },
     {
-      "core": true,
+      "core": false,
       "difficulty": 1,
-      "slug": "two-fer",
+      "slug": "reverse-string",
       "topics": [
-        "conditionals",
         "strings"
       ],
       "unlocked_by": null,
-      "uuid": "74515d45-565b-4be2-96c4-77e58efa9257"
+      "uuid": "2c8afeed-480e-41f3-ad58-590fa8f88029"
     },
     {
       "core": false,