Do not declare #value as an attribute method.

Some revision between ActiveRecord 3.1 and 3.2 introduced a requirement
that attribute methods in ActiveRecord::Base descendants be backed by
columns.

I'm not sure what revision that is because I'm not interested enough to
bisect Rails for this, but I do know that you get a stack trace like
this without this commit:

    activerecord-3.2.13/lib/active_record/attribute_methods/time_zone_conversion.rb:60:in `create_time_zone_conversion_attribute?':
        undefined method `type' for nil:NilClass (NoMethodError)
    from activerecord-3.2.13/lib/active_record/attribute_methods/time_zone_conversion.rb:24:in `attribute_cast_code'
    from activerecord-3.2.13/lib/active_record/attribute_methods/serialization.rb:80:in `attribute_cast_code'
    from activerecord-3.2.13/lib/active_record/attribute_methods/read.rb:72:in `define_method_attribute'
    from activerecord-3.2.13/lib/active_record/attribute_methods/primary_key.rb:29:in `define_method_attribute'
    from activemodel-3.2.13/lib/active_model/attribute_methods.rb:267:in `block in define_attribute_method'
    from activemodel-3.2.13/lib/active_model/attribute_methods.rb:260:in `each'
    from activemodel-3.2.13/lib/active_model/attribute_methods.rb:260:in `define_attribute_method'
    from /Users/davidyip/Projects/ncs/core/lib/ncs_navigator/core/surveyor/response_value.rb:28:in `included'

The frame at the ActiveModel -> ActiveRecord transition point (i.e.
@ primary_key.rb:29) is executing code that was introduced in Rails @
e9fb6d04bdfbe3dfc4ba1a3cf442ec5d25764300.  Again, that's not
establishing causation, which means that this whole paragraph is really
worthless, but just in Case You Might Want To Know, etc.

More motivated detectives are welcome.