Use ruby's File::Separator rather than '/' as a default for directory_separator

[swiknaba]

Issue#

No-Story

Description:

Replace '/' with File:: Separator from Ruby kernel as a default directory_separator for File.dir and File.file_name to be more OS-agnostic out of the box.
AFAIK, this constant should exist at least since Ruby 2.1.

Test:

from the guide:

Please add a test for your change. Only refactoring and documentation changes require no new tests. If you are adding functionality or fixing a bug, we need a test! We use Minitest in this project.

for my feeling, this is just a small refactoring, thus I did not explicitly add a test.

Running test locally:

# tests
1458 tests, 81210 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed

# rubocop
427 files inspected, no offenses detected

[koic]

This change seems to make sense 👍