Compatibility with guzzle 7.

[donquixote]

The ClientInterface::VERSION no longer exists in Guzzle 7. This PR is a proof of concept, I do not claim that this is the ultimate way to deal with this.

I accidentally created this in the wrong repo first: https://github.com/Behat/MinkExtension/pull/387. I now see that this repo is more up to date, and is in fact what I am using in my project.

[Yozhef]

@donquixote please update the branch to start the ci process