Update PHPUnit from 4.X to 6.X in acceptance tests

The PHPUnit update also required an update of "symfony/yaml", so besides
the changes needed for PHPUnit the "behat.yml" file also had to be
adjusted.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

Author: danxuliu

tests/acceptance/composer.json

@@ -4,7 +4,7 @@
     "behat/mink": "1.7.1",
     "behat/mink-extension": "2.3.1",
     "behat/mink-selenium2-driver": "1.3.1",
-    "phpunit/phpunit": "4.8.36"
+    "phpunit/phpunit": "6.5.14"
   },
   "autoload": {
     "psr-4": {