Allow PhpUnit version 5

Author: sroze

composer.json

@@ -23,7 +23,8 @@
 
     "require-dev": {
         "mink/driver-testsuite": "dev-master",
-        "symfony/http-kernel": "~2.3|~3.0"
+        "symfony/http-kernel": "~2.3|~3.0|~4.0",
+        "phpunit/phpunit": "^4.8.35|^5.7"
     },
 
     "autoload": {