Support for Symfony 4

[sroze]

Use this opportunity to also drop Symfony 2.3, replace 3.0 with 3.3 in Travis.

Depends on:

• [x] https://github.com/Behat/MinkExtension/pull/308
• [x] https://github.com/minkphp/MinkBrowserKitDriver/pull/123
• [x] https://github.com/minkphp/Mink/pull/749

[tyx]

I'm available to help on this subject as this PR is the latest requirement to bump to symfony 4 on our side ^^

[sroze]

I believe the last PR is the one on minkphp/MinkBrowserKitDriver#123

[sroze]

Merged and relased 2.1.3 🎉

[ruudk]

@sroze You rock! 💪