Move selenium2 driver to ss vendor

See https://github.com/Behat/MinkSelenium2Driver/issues/153 for the cause of this issue.

Upgrading to behat 3.0 is not impossible, but is a lot of work and necessary investment.

Downgrading to a behat 2.5 compatible version using the thirdparty vendor would require us to use 1.1.1 of mink-selenium2-driver, which is 1 year old and suffers from bugs.