Backward compatibility for the Loggable mixin which is not available on Logstash 2.4.1
In your Logstash plugin gemspec add/change
s.add_runtime_dependency "logstash-core-plugin-api", ">= 1.60.5", "<= 2.99"
s.add_runtime_dependency 'logstash-loggable-bwc'
Then in you plugin replace any
require "logstash/util/loggable"
with
require "logstash-loggable-bwc"