Drop rubysl dependency for Rubinius/RBX
Drop rubysl dependency for Rubinius/RBX

It hasn't been updated for 3 years, and isn't compatible with Ruby 3.
Furthurmore, the specs for RBX were dropped at
https://github.com/mongomapper/mongomapper/commit/48c31ca6053d9536378ae69f728e9ea32309b03f.

mongomapper/mongomapper