jls-lumberjack gem to 0.0.20 with version frame to allow backward compatibility

Super quick patch to add backwards compatibility to the server module.
Future logstash-forwarder can send "1V<proposed_version>" and server will return "1V<version>" (same or lower than proposed.) Version before frame char couldn't be used for this - send an unknown frame and your connection ends.

Be good to get in LogStash sooner than later to open an easy road for protocol improvements without breaking stuff - if we change protocol though (#119) it becomes irrelevant but if we don't... I think this is important!