Update README about rails3 branch

Update README.md

Add a mention for https://github.com/radar/paranoia/issues/109#issuecomment-74455252

I think this advice is misleading for most datasets and most RDBMS this
is not necessary, and as per #206 doesn't even work.

I have moved it to the wiki
https://github.com/radar/paranoia/wiki/Custom-sentinel-values

This is a test for a case when a model has a has_one association with an
after_validation callback, and we have failed validation. Paranoia will
attempt to touch the deleted_at column on the child record that has
been built but not saved.

Fixes #180

This allows testing rails 4.0 on ruby 2.2

We no longer have different behaviour between AR versions.

We shouldn't be overriding this. soft-deleted objects are still
persisted.

The default implementation is already destroy || raise

Thanks @kidlab

There was a change in ActiveRecord#update_columns on latest rails
so now it checks for `destroyed?` instead of `persisted?`. Paranoia
can't override `destoyed?` like this anymore otherwise
`update_columns` will always raise:

        ActiveRecord::ActiveRecordError: cannot update a destroyed record

see https://github.com/rails/rails/commit/0f99aa615e11c50cc64b567f54cf64f59108c5e5

To ensure that we are using the association _type in our scope.

If the association object is a has_one relationship with an :as option,
it will have a type attribute (see
https://github.com/rails/docrails/blob/master/activerecord/lib/active_record/reflection.rb#L280).

If this type attributes is present, that will be the type column on the
polymorphic model. The foreign key can be found as an attribute on
the association object.

Restoring polymorphic has_one relationships errored because paranoia was
not correctly looking up the foreign_key.

Output from failing test -

ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: parent_model_id: SELECT  "polymorphic_models".* FROM "polymorphic_models" WHERE ("polymorphic_models"."deleted_at" IS NOT NULL) AND (parent_model_id)  ORDER BY "polymorphic_models"."id" ASC LIMIT 1

The test sets up a PolymorphicModel, which has_many parents. The
ParentModel then has a has_one relationship with PolymorphicModel. When
restoring, the foreign key is set as - `self.class.name.to_s.underscore_id`
which will be parent_model_id, instead of the :as option.

DEPRECATION WARNING: Currently, Active Record suppresses errors raised within `after_rollback`/`after_commit` callbacks and only print them to the logs. In the next version, these errors will no longer be suppressed. Instead, the errors will propagate normally just like in other Active Record callbacks.

apply the deprecation fix for ActiveRecord >= 4.2 only

DEPRECATION WARNING: You are passing an instance of ActiveRecord::Base to `find`. Please pass the id of the object by calling `.id`. (called from block in restore at /Users/sergey/ruby_projects/paranoia/lib/paranoia.rb:38)

😒

Fix matcher compatibility with RSpec 2