Rails 4.2 support

We still have a broken test due escaping, probably similar to what is going on with the Simple Form test suite (https://github.com/plataformatec/simple_form/issues/1130). Beside that, I think the `raise_on_spam` flag could use some better naming and some docs :smile:.
to inject the `raise_on_spam` config instead of having the `defined?(Rails) && ...` call inside the `delivery.rb` file.