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:.
Ah, so we still need to implement this config in the railtie right? Because right now we don't need it. Since we can do `MailForm.raise_on_spam = true` in development.rb