rails/rails — comment on commit dad7fdc5734a3813246f238ac5760b2076932216
File:  line 0
By drogus:

So to sum it up - it's still dead easy to have vendor/plugins functionality, but without any additional code, just using bundler. And you can quickly push such gem to rubygems without any changes (well, almost without any changes, you may need to set a few things in a gemspec).