Updating ffi-rzmq

When testing logstash with zeromq input plugin, there was an incompatibility with libzmq, version 4.0.4:

./logstash agent -e "input { zeromq { topology => 'pubsub' mode => 'client' address => 'tcp://<ip>:<port>' } } output { stdout{} }"

Using milestone 2 input plugin 'zeromq'. This plugin should be stable, but if you see strange behavior, please let us know! For more information on plugin milestones, see http://logstash.net/docs/1.4.1/plugin-milestones {:level=>:warn}
LoadError: The libzmq version 4.0.4 is incompatible with ffi-rzmq.

This issue makes it impossible to use logstash with newer versions of 0MQ. Updating ffi-rzmq gem to version 2.0.1, which supports 0mq 4.x API, seems to solve the problem with zeromq plugin.