Rename Paranoia#destroyed? to Paranoia#paranoia_destroyed?

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 rails/rails@0f99aa6

@radar this is a breaking change so not sure how you want to handle it. Couldn't think of a fix other than renaming the method. I'm not sure why `destroyed?` was overridden here in the first place but if tests are missing a scenario this could break something else too. Let you know if I find anything new.