Ruby-core-source doesn't handle Ruby 2.0 sources

[ericpromislow]

Two problems:

1. It assumes the tar.gz's are still in a 1.9/ parent dir.
2. For reasons I can't figure, when I run the code under Ruby 2.0.0-p0,
   the .tar.gz file comes down as a plain tar file.

Pull request on its way.