Fix Pruner not deleting nested empty dirs

`Nanoc::Extra::Pruner` won't delete nested empty dirs, e.g. `output/a/b/c`, because the dirs are traversed in top-down order (`output/a`, `output/a/b`, `output/a/b/c`). Only `output/a/b/c` would be deleted in this case, because `output/a` and `output/a/b` were found to be non-empty before.

Iterating over the dirs in reverse order fixes this problem, as subdirs are now handled before their parent dirs.