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 require is problematic on an existing site.