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:.
damn, I've forgot to push the rest of the code :runner:. But yeah, we could remove the railtie and leave this directly in the env files, but that would be a manual step in the upgrade path.