Ruby testing on stable debian server
See: https://stackoverflow.com/questions/512906/debian-how-can-i-pull-a-single-package-with-dependencies-from-another-repository/513078#513078
The mechanics are exactly the same despite that having been written for etch using packages from lenny. As a further suggestion, always use the name of the distribution rather than 'stable' as to avoid being bitten by an unplanned upgrade around release time. Feel free to use 'testing' though.