Use enum_for with ascend in rubocop_yml_for

It turns out that even though `Pathname#ascend` wasn't written to return
an enumerator when no block was given, you can use a handy-dandy helper
called `enum_for` to wrap the call in an enumerator.

This allows the method to be drastically simplified to use `map` and
`find`.

Change-Id: Idda85f38ffd0e683334a20cdd114c5d4234d8d66
Reviewed-on: https://gerrit.causes.com/30175
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Lann Martin <lann@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>