Fix a bug where s3 treats the version "1-1" less than "1"

When results are returned by the call to List, they are sorted in order of
common prefix, which includes the trailing slash. Since "/" sorts higher than
"-", this was causing Sequins to pick up the wrong versions sometimes.

The new behavior, while not exactly consistent with s3's worldview, is
consistent with the other backends - and we were already treating s3 as a fake
filesystem anyway.