Fix cactus_preprocessor interaction with s3:// URLs
Fix cactus_preprocessor interaction with s3:// URLs

I would fix it for other URLs, but these seem to be the only writable
URLs supported by toil right now.

benedictpaten/cactus,benedictpaten/cactus,benedictpaten/cactus,benedictpaten/cactus,benedictpaten/cactus