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.