Selenium drivers do not support response header checking (http://mink.behat.org/en/latest/guides/drivers.html) which are useful to develop http-cache tests for Varnish (see: https://github.com/ezsystems/BehatBundle/pull/86)
It is possible using Chromium driver.
Goal of this task:
1) add support for Chromium in our Docker stack
2) add Chromium as an additional Mink driver
3) Run whole regression suite on Chromium on Travis and evaluate whether we should switch to it and abandon Selenium