small style improvement in ImmediateExecutor

Author: mighe

lib/concurrent/executor/immediate_executor.rb

@@ -4,7 +4,7 @@ class ImmediateExecutor
     def post(*args, &task)
       raise ArgumentError.new('no block given') unless block_given?
       task.call(*args)
-      return true
+      true
     end
 
     def <<(task)