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)
```