To fix to dependencies tested on Drupal 8.4.0 add "webflo/drupal-core-strict": "8.4.0" to the require section of your composer.json and run composer update from the command line.

In other words: this is a virtual package, that causes you to get exactly the versions of Drupal core's dependencies as they are specified in Drupal core's composer.lock file.

Using composer require is problematic on an existing site.