Fix reference to mink:symfony2
The correct tag for the Symfony2Extension has been `@mink:symfony2` rather than `@mink:symfony` since https://github.com/Behat/Symfony2Extension/commit/ff6a1e590e421cabcb1f7afd99ad6b01c3b731b6
Merge pull request #46 from rh389/fix-tag

Fix reference to mink:symfony