Use PHPUnit 6.0 on PHP 7.* test lines

Author: nicolas-grekas

composer.json

@@ -23,7 +23,7 @@
         "symfony/event-dispatcher": "~2.5",
         "symfony/finder": "^2.0.5",
         "symfony/http-foundation": "~2.7",
-        "symfony/http-kernel": "~2.7.23|^2.8.16",
+        "symfony/http-kernel": "~2.7.25|^2.8.18",
         "symfony/filesystem": "~2.3",
         "symfony/routing": "~2.7.24|^2.8.17",
         "symfony/security-core": "~2.6.13|~2.7.9|~2.8",
@@ -40,7 +40,7 @@
         "symfony/dom-crawler": "^2.0.5",
         "symfony/intl": "~2.3",
         "symfony/security": "~2.6",
-        "symfony/form": "~2.7.23|^2.8.16",
+        "symfony/form": "~2.7.25|^2.8.18",
         "symfony/class-loader": "~2.1",
         "symfony/expression-language": "~2.6",
         "symfony/process": "^2.0.5",