It used to work a few days ago, but as of today there is 7.x version of drupal/redis. Off course, there is a 7.x release on drupal.org.
$ composer update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.
Problem 1
- The requested package drupal/redis ^7.3 exists as drupal/redis[8.1.0-alpha1, dev-8.x-1.x, 8.1.x-dev] but these are rejected by your constraint.
It used to work a few days ago, but as of today there is 7.x version of drupal/redis. Off course, there is a 7.x release on drupal.org.