Add concurrent-ruby dependency.

Author: theturtle32

lib/kafka/version.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module Kafka
-  VERSION = "1.1.0.beta1"
+  VERSION = "1.1.0.beta2"
 end

ruby-kafka.gemspec

@@ -28,6 +28,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.add_dependency 'digest-crc'
+  spec.add_dependency 'concurrent-ruby', '~> 1.0', '>= 1.0.2'
 
   spec.add_development_dependency "bundler", ">= 1.9.5"
   spec.add_development_dependency "rake", "~> 10.0"