Removed Stoppable, which is no longer being used.

Author: jdantonio

lib/concurrent.rb

@@ -24,7 +24,6 @@
 require 'concurrent/promise'
 require 'concurrent/runnable'
 require 'concurrent/scheduled_task'
-require 'concurrent/stoppable'
 require 'concurrent/supervisor'
 require 'concurrent/timer_task'
 require 'concurrent/tvar'