Merge pull request #438 from exercism/isogram-rate

isogram: rate difficulty 4, reposition

config.json

       "topics": []
     },
     {
+      "slug": "isogram",
+      "difficulty": 4,
+      "topics": []
+    },
+    {
       "slug": "flatten-array",
       "difficulty": 5,
       "topics": []
       "difficulty": 1,
       "topics": []
     },
-     {
-      "slug": "isogram",
-      "difficulty": 1,
-      "topics": []
-    },
     {
       "slug": "book-store",
       "difficulty": 1,