Merge pull request #463 from ruby-concurrency/bug/map-native-extensions

Fixed bug with requires for Map.

Author: jdantonio

lib/concurrent/map.rb

@@ -1,7 +1,6 @@
 require 'thread'
 require 'concurrent/constants'
-require 'concurrent/utility/native_extension_loader'
-Concurrent.load_native_extensions
+require 'concurrent/synchronization'
 
 module Concurrent
   # @!visibility private