Input plugin runtime dependency

Another thing I cannot figure out.

I have developed a logstash input plug-in but during installation I now have a depedency to logstash-core 5.... Why?

My gemspec file contains:
s.add_runtime_dependency "logstash-core", ">= 2.0.0", "< 3.0.0"
s.add_runtime_dependency 'logstash-codec-plain'

And suddenly this requires an alpha version of a new logstash-core gem...
What is the correct runtime dependency configuration for working the the lastest STABLE logstash gems?