FYI you don't need "behat/mink-extension": "dev-master" any more.
"behat/mink-extension": "dev-master"
It works for me with:
"behat/behat": "3.*@stable","behat/mink": "1.6.*@stable","behat/mink-extension": "@stable","behat/mink-goutte-driver": "@stable",