Removes broken attribute interpolation

Dough's idea of interpolating model attribute names into the validation
messages was unfortunately somewhat misleading.

By default rails *does not* include the attribute name in the message. You can
however retrieve the message with attribute name by requesting
`errors#full_messages` or `errors#full_messages_for(:attrib)`.

If the expected behaviour is that we always return the full message including
attribute name we should respect rails' approach rather than butchering the
locale files to achieve the same ends. This PR brings us back to that behaviour
but may well introduce breaking changes to code that depended on it prior hence
the significant version bump.

If you need entirely custom error messages, that is - messages without their
associated attribute name interpolated - you should add your errors to `:base`