Require a stable code coverage version to support HHVM

Author: wouterj

composer.json

@@ -23,6 +23,7 @@
         "symfony-cmf/testing": "~1.2,>=1.2.6",
         "matthiasnoback/symfony-dependency-injection-test": "~0.6",
         "matthiasnoback/symfony-config-test": "0.*",
+        "phpunit/php-code-coverage": "@stable",
         "sonata-project/doctrine-phpcr-admin-bundle": "~1.1",
         "symfony/monolog-bundle": "~2.3",
         "doctrine/orm": "~2.3",