Use Pathname.ascend in rubocop_yml_for

Using `Pathname.ascend` is a little more conventional. Also updated the
changelog to mention the fix in 2119b447.

Change-Id: I7a8f8bdd61f64bda3056dda72102b47da7fc2146
Reviewed-on: https://gerrit.causes.com/30172
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>