Mongoid 3.0 - Fix the to_json serializable hash duplication issue

This small change to serliazable_hash will fix part of the issue described in https://github.com/jnicklas/carrierwave-mongoid/pull/59

Please note that it does not fix all of the failing specs submitted by NVDK, just the one that relates to the to_json serialization returning the wrong info when multiple uploaders are used. 

The crux of the issue is that serializable_hash was being defined each time mount_uploader is called, with each successive call overwriting the method definition. This wouldn't be a big issue, except that the method uses :#{column} with interpolation. I have changed this to use column.to_sym, which ensures that at runtime the _mouter gets the right column name. 

As I fix the rest of the specs, I am thinking that serializable_hash may need to come out of the class_eval block. 

Does that sound right to you?