You might have this happen to you if you switch back to an old project using Homestead. Basically you have a new version of the laravel/homestead vagrant box (1.0.1) which includes php 7.1 but the package in composer ^3.0 is expecting php 7.0.

Solution

Just add the line version: 0.6.0 to your homestead.yaml to indicate you want that version for the vagrant box which is the last one that includes php 7.0.