Hi @ben
Nevermind i have managed to fix my problem with brew using this ressource :
brew update && brew upgrade
brew tap homebrew/dupes
brew tap homebrew/versions
brew tap homebrew/homebrew-php
brew unlink php56
brew install php71
than in your terminal, type:
export PATH=/usr/local/php5/bin:$PATH
now i have php 7.1 version on my system and the following command works
composer create-project roots/sage your-theme-name dev-master
Hope it helps someone else