webflo/drupal-core-require-dev provides the require-dev dependencies of drupal/core as a standalone package. It follows the same release cycle and versioning scheme as Drupal core. You should use the same version constraint for it as you use for Drupal core.
composer require --dev webflo/drupal-core-require-dev ~8.4.0
composer require --dev webflo/drupal-core-require-dev ~8.5.0