Dalli security warning

[philayres]

The log displays the following during startup:

SECURITY WARNING: Dalli is using Marshal for serialization. Marshal can execute arbitrary code during deserialization. If your memcached server could be compromised, consider using a safer serializer like JSON: Dalli::Client.new(servers, serializer: JSON)

If our memcached server is compromised, the whole server has been compromised. Dalli doesn't amplify the issues in this case.

To resolve, this warning should be squashed somehow.