Bootstrap 4 is out of alpha now...

I don't want rjsf to have to support too much. Ideally we could support either one or zero HTML/CSS frameworks "by default", and the other ones could be added by users in other packages or something. But I think the fact that Bootstrap v3 is now deprecated means the one we support "by default" shouldn't be that. On the other hand, moving unilaterally to a new Bootstrap might be a breaking change for our users. Or maybe it wouldn't be so bad, because it's just a version bump on a dependency in package.json. Thoughts?