Allow Symfony 4 as dependency

[Berdir]

Same as jhedstrom/DrupalDriver#222

[Berdir]

Not sure what exactly the problem is here. Only thing I found was symfony/symfony#29907, could be a conflict between what is used by drupal and what's added by this, because I think we're not actually installing drupalextension inside drupal and then nothing ensures that version are kept in sync.

[Berdir]

To make this work and install the proper version of things, we might want to use a similar approach as DrupalCI, and add the git checkout of drupalextension as a repository to drupal an then install it from there, that will then respect existing dependencies and pick a compatible version.

[Berdir]

Got it to pass with a workaround and created #570 with ideas on how to improve travis testing.

[MPParsley]

Looks good! We also need Symfony 4 support.

$ composer why symfony/browser-kit
behat/mink-browserkit-driver  1.3.3   requires  symfony/browser-kit (~2.3|~3.0|~4.0)  
drupal/drupal-extension       v4.0.1  requires  symfony/browser-kit (^3.4)            
fabpot/goutte                 v3.2.3  requires  symfony/browser-kit (~2.1|~3.0|~4.0)