Add require 'fiber' for Fiber.current usage.

Fixes <#962>.

Author: ioquatix

lib/concurrent-ruby/concurrent/atomic/locals.rb

@@ -1,6 +1,8 @@
 require 'concurrent/utility/engine'
 require 'concurrent/constants'
 
+require 'fiber'
+
 module Concurrent
   # @!visibility private
   # @!macro internal_implementation_note