only remove const if it is defined

Author: tenderlove

lib/syck.rb

@@ -11,7 +11,7 @@
     require File.expand_path('../yaml/engine_manager', __FILE__)
 else
     Object.class_eval <<-eorb, __FILE__, __LINE__ + 1
-        remove_const 'YAML'
+        remove_const 'YAML' if defined? YAML
         YAML = Syck
         remove_method :to_yaml
         alias :to_yaml :syck_to_yaml