Quick fix for rubinius until rubysl gets updated

Thank you for addressing issues so quickly! Unfortunately, the latest work (https://github.com/thekompanee/chamber/issues/11) broke Rubinius compatibility again. I already created a pull request to the rubysl (https://github.com/rubysl/rubysl-pathname/pull/2), but my guess is this is going to take a while. Therefore I created this little fix.