Merge pull request #505 from ruby-concurrency/fix-timer-task

TimerTask specifies all its dependencies.

Author: jdantonio

lib/concurrent/timer_task.rb

@@ -3,7 +3,9 @@
 require 'concurrent/concern/observable'
 require 'concurrent/atomic/atomic_boolean'
 require 'concurrent/executor/executor_service'
+require 'concurrent/executor/ruby_executor_service'
 require 'concurrent/executor/safe_task_executor'
+require 'concurrent/scheduled_task'
 
 module Concurrent