So far, the author writes, it only supports 1.8 and not all of standard library. But if it ever supports 2.4 + Rails, that would be enormous.
Yes, it is because this is a fork of a project started in 2009 (and not maintained afterward). I spent a couple of evenings trying to resurrect it and see what this thing is capable of. There is a lot of things to do to make this really usable. But I was able to get it to the point to run examples (those examples are the same in ruby 2.4).