FIX: Remove unnecessary dependencies.
FIX: Remove unnecessary dependencies.

These composer dependencies were inlined from Behat in an attempt to 
lock in PHP 5.3-compatible versions of Symfony packages. This is no
longer relevant and so we can remove this.
silverstripe-labs/silverstripe-behat-extension,tractorcow/silverstripe-behat3,silverstripe/silverstripe-behat-extension,silverstripe/silverstripe-behat-extension