don't need explicit dep and a pretty neat pick

actioncable/actioncable.gemspec

@@ -23,7 +23,6 @@ Gem::Specification.new do |s|
   s.add_dependency 'coffee-rails',     '~> 4.1.0'
   s.add_dependency 'faye-websocket',   '~> 0.10.0'
   s.add_dependency 'websocket-driver', '~> 0.6.1'
-  s.add_dependency 'concurrent-ruby',  '~> 1.0.0'
   s.add_dependency 'em-hiredis',       '~> 0.3.0'
   s.add_dependency 'redis',            '~> 3.0'
 

actioncable/lib/action_cable/server/worker.rb

@@ -12,7 +12,7 @@ module ActionCable
       define_callbacks :work
       include ActiveRecordConnectionManagement
 
-      def initialize(max_size=5)
+      def initialize(max_size: 5)
         @pool = Concurrent::ThreadPoolExecutor.new(
           min_threads: 1,
           max_threads: max_size,