change a difficulty of 0 to an allowed value (#325)

Previously, a difficulty value of 0 was allowed, and `configlet lint`
checked that each difficulty value was in the range from 0 to 10
inclusive.

However, a difficulty of 0 is now disallowed. Typically, exercises with
a difficulty value of 0 were deprecated exercises, but these still
appear on the website for users who started them before they were
deprecated, and so they should still have a difficulty level as normal.

With this commit, we change the difficulty of our one deprecated
exercise (`binary`) to an allowed value.

See:
- exercism/docs#290
- exercism/configlet#447

Author: ErikSchierboom

config.json

@@ -325,7 +325,7 @@
         "uuid": "3acf148d-df9e-4897-8c9f-c17a3ba5e4b6",
         "practices": [],
         "prerequisites": [],
-        "difficulty": 0,
+        "difficulty": 1,
         "topics": null,
         "status": "deprecated"
       },