Fix failing tests due to symfony/yaml autoloading

- phpunit/phpunit was relying on functionality from symfony/yaml that was
  removed in version 2.2.0; pinning symfony/yaml to 2.1.* solves those issues.

composer.json

@@ -13,7 +13,8 @@
     },
     "require-dev": {
         "doctrine/common": ">=2.1",
-        "phpunit/PHPUnit": "3.7.*"
+        "phpunit/PHPUnit": "3.7.*",
+        "symfony/yaml": "2.1.*"
     },
     "suggest": {
         "doctrine/common": "Doctrine\\Common >=2.1 for annotation features",