Require at least PHPUnit bridge 2.8.1

It's the first release with the deprecations upper bound feature included: symfony/symfony#16937

Author: wouterj

composer.json

@@ -12,7 +12,7 @@
     "require": {
         "php": "^5.3.9|^7.0",
         "symfony/symfony": "^2.3|3.*",
-        "symfony/phpunit-bridge": "^2.7|3.*",
+        "symfony/phpunit-bridge": "^2.8.1|3.*",
         "symfony/monolog-bundle": "^2.1|3.*",
         "doctrine/common": "^2.4",
         "doctrine/doctrine-bundle": "^1.0",