get rid of AS attribute_accessors deprecation message.

The file was moved and we need to require a different path.
This change should only apply to Rails 4.1 and above.

This will eliminate the following deprecation message:

```
DEPRECATION WARNING: The cattr_* method definitions have been moved into active_support/core_ext/module/attribute_accessors. Please require that instead. (called from require at /Users/senny/Projects/protected_attributes/test/abstract_unit.rb:3)
```

test/abstract_unit.rb

@@ -1,8 +1,17 @@
 require 'action_dispatch'
 require 'action_controller'
-require 'active_support/core_ext/class/attribute_accessors'
 require 'active_support/dependencies'
 
+def active_support_4_0?
+  ActiveSupport::VERSION::MAJOR == 4 && ActiveSupport::VERSION::MINOR == 0
+end
+
+if active_support_4_0?
+  require 'active_support/core_ext/class/attribute_accessors'
+else
+  require 'active_support/core_ext/module/attribute_accessors'
+end
+
 module SetupOnce
   extend ActiveSupport::Concern