bump `packedjson` dependency to include a fix for ARC/ORC (#691)

Hello. `shallowCopy` has been removed for ARC/ORC since it does a deep copy for strings/seqs, which breaks the semantics of `shallowCopy`. https://github.com/Araq/packedjson/pull/13 is a fix for `packedjson` to support ARC/ORC. The PR bumps `packedjson` dependency to include [a fix](https://github.com/Araq/packedjson/commit/9e6fbb63cb9e0ab8ff570c04bec28cc713ca1f7c) for ARC/ORC.

nitter.nimble

@@ -16,7 +16,7 @@ requires "karax#5498909"
 requires "sass#e683aa1"
 requires "nimcrypto#a5742a9"
 requires "markdown#a661c26"
-requires "packedjson#d11d167"
+requires "packedjson#9e6fbb6"
 requires "supersnappy#2.1.1"
 requires "redpool#8b7c1db"
 requires "https://github.com/zedeus/redis#d0a0e6f"