Fix Selenium2 Driver dependency

  Problem 1
    - Installation request for behat/mink-selenium2-driver 1.2.x-dev -> satisfiable by behat/mink-selenium2-driver[1.2.x-dev].
    - behat/mink-selenium2-driver 1.2.x-dev requires behat/mink ~1.6@dev -> no matching package found.