index/schema.py: Make cmd_version actually optional.

During migration, we may have to import json index file which was
produced with an older version. If the index file is missing
cmd_version, migration will fail.

archivebox/index/schema.py

@@ -59,6 +59,7 @@ class ArchiveResult:
         }
         info['start_ts'] = parse_date(info['start_ts'])
         info['end_ts'] = parse_date(info['end_ts'])
+        info['cmd_version'] = info.get('cmd_version')
         return cls(**info)
 
     def to_dict(self, *keys) -> dict: