Increase lumberjack gemspec version for protocol backwards compatibility fixes

Hi,

Currently if the lumberjack protocol is improved (new frame types etc) there is no way for a client to tell. It means no backwards compatibility possible. 

The counterpart to this which is required is there: https://github.com/elasticsearch/logstash-forwarder/pull/173
Someone should check I'm doing the gem bit right though... gem-n00b sorry.

The patch itself is minor but would mean if logstash-forwarder was improved and protocol was improved, it would still work out of the box as of the version this is merged to. 1.4.0 sounds good :-) But it would mean we don't worry about new logstash-forwarder crashing the "current" LogStash - I think it important the current LogStash can work with future logstash-forwarder, for user experience.

Thanks! All feedback, good and bad, welcome.

Jason

FYI: I am currently working on a new protocol for logstash-forwarder, specifically partial ACKs and such - and also improving the server gem so redis congestion doesn't cause "Too many open files" crashes or constant disconnects etc. and the connection gracefully waits - most is done just testing and tweaking.