Add the ability to configure which firefox / chrome versions (selenium images) we want to use

[stronk7]

Recently we have started to run behat tests using dockerized browser + selenium. Right now, and until we get the webdriver drives updated to newer W3C ones in behat... we are using super-old Firefox 2.53.1 and, for Chrome, we are running on the edge (latest).

Whenever we change the drivers to the new ones, or if there are breaking changes in chrome (has happened a number of times), of if somebody wants to run the tests against older versions, it should be possible to specify/pass the exact version to use (or maybe whole image if a custom one, say orangehq/super-selenium:99.99, not sure, just sharing the thought).

This was created while reviewing #13.