Refactoring of #get_disk_reference to fix symlink issue on JRuby.

There's an issue with resolving symlinks in #get_disk_reference caused by a known bug in Pathname.realpath in [JRuby](https://github.com/jruby/jruby/issues/538). This fix uses ::File.readlink to resolve the symlink instead. It works with MRI and JRuby.