fixes the breaking change introduced by 0.30.0

```
Error: abstract `def Enumerable(T)#each(&block)` must be implemented by Callback::ResultSet(T)
```

this fixes a transient dependency of optarg which is not compatible with
0.30.0.
we use the fork until mosop/callback#3 has been
merged and optarg has been updated to that version.

Author: koffeinfrei

shard.lock

@@ -5,7 +5,7 @@ shards:
     version: 0.10.0
 
   callback:
-    github: mosop/callback
+    github: drujensen/callback
     version: 0.6.3
 
   cli:

shard.yml

@@ -8,6 +8,12 @@ description: |
   Simple CLI to ask the internet if it's worth watching this movie.
 
 dependencies:
+  # FIXME: Fixes the breaking change introduced by 0.30.0:
+  #   Error: abstract `def Enumerable(T)#each(&block)` must be implemented by Callback::ResultSet(T)
+  # Use the fork until https://github.com/mosop/callback/pull/3 has been merged
+  # and optarg has been updated to that version.
+  callback:
+    github: drujensen/callback
   myhtml:
     github: kostya/myhtml
   crest: