Handle activemodel::missingattributeerror on select in activerecord4

From the activerecord 4 documentation:

> Accessing attributes of an object that do not have fields retrieved by a select will throw 
> ActiveModel::MissingAttributeError:

> Model.select(:field).first.other_field
> ActiveModel::MissingAttributeError: missing attribute: other_field

Which breaks compatibility of this gem with rails 4. This pull request fixes the issue.