Increase ffi-rzmq dependency version to 2.0.x, minimum 2.0.4

Fixes #1

logstash-input-zeromq.gemspec

@@ -1,7 +1,7 @@
Gem::Specification.new do |s|

  s.name            = 'logstash-input-zeromq'
  s.version         = '0.1.3'
  s.version         = '0.1.4'
  s.licenses        = ['Apache License (2.0)']
  s.summary         = "$summary"
  s.description     = "This gem is a logstash plugin required to be installed on top of the Logstash core pipeline using $LS_HOME/bin/plugin install gemname. This gem is not a stand-alone program"
@@ -23,7 +23,6 @@ Gem::Specification.new do |s|
  s.add_runtime_dependency "logstash-core", '>= 1.4.0', '< 2.0.0'

  s.add_runtime_dependency 'logstash-codec-json'
  s.add_runtime_dependency 'ffi-rzmq'
  s.add_runtime_dependency 'ffi-rzmq', '~> 2.0.4'
  s.add_development_dependency 'logstash-devutils'
end