Execute Behat scenarios by tag groups on Travis CI

@pjedrzejewski I noticed that you introduced tags for the Behat features and I assume you execute them tag group by tag group in order to avoid the PHP memory issue on Travis CI (eg. https://travis-ci.org/Sylius/Sylius/jobs/10005147#L586). Thus I changed Sylius' `.travis.yml` to do exactly that by default (see https://travis-ci.org/headrevision/Sylius/jobs/10026602).