Logstash[ version-1.5.4] stopped if I insert lumberjack input in configuration

Not sure whether I can post this issue here. as I have posted the same on github.

Problem : When I insert lumberjack configuration in logstash-1.5.4 , its getting stopped with this error message " The error reported is: \n uninitialized constant Concurrent::Delay::Executor". Do anyone have idea on this ?.

upgrade 1.0.5
./bin/plugin update logstash-input-lumbjerjack