Bump version to 0.0.9

I’m all for [releasing version 1.0.0](https://github.com/dkubb/equalizer/issues/14), but before we do that, we should fix the current release, which is broken on Ruby 1.8.7 and has requires dependent gems that support Ruby 1.8.7 to do [something like this](https://github.com/sferik/twitter/commit/2b91c9c7096697c1f79f22626782bd13207693ac). :grimacing:

As part of this pull request I’ve also added a statement to the `README` on "Supported Ruby Versions" (this can be updated to remove Ruby 1.8.7 and REE before the 1.0.0 release).

I’ve also taken the liberty of adding myself to the ”Credits” section of the `README`, as I will be maintainer and release manager for the `0.x` branch. As such, I would request to be added as a collaborator on this project, so I can push 0.x releases without needing to submit pull requests for version bumps. :wink: