Rebirth 0.1 is done. All it can do is open and close the View (equivalent to Screen in Rubygame — in fact, built on top of Screen). So, you probably don't want or need to download it. It's immortalized as tag release-0.1 on Github.
0.2 will be done when keyboard events are done. 0.3 will add a Rectangle class, so there will actually be something to see. You can read the ROADMAP to see the rest of the releases that are currently planned (up to 0.8 so far).