Provide a stable phpunit version ~5.3 for every unittest

This is needed for everyone writing unit tests for this bundle. When setting up new TestCases, it is a must, that you know on which phpunit version you can rely on.

composer.json

@@ -45,6 +45,7 @@
         "jms/di-extra-bundle": "~1.7",
         "sensio/generator-bundle": "~2.3|~3.0",
         "symfony/yaml": "~2.3|~3.0",
+        "phpunit/phpunit": "^5.3",
         "sonata-project/intl-bundle": "^2.2.4",
         "symfony/phpunit-bridge": "~2.7|~3.0"
     },