Fix failing test on composer with --prefer-lowest flag

composer.json

@@ -33,7 +33,7 @@
         "twig/twig": "~1.23",
         "twig/extensions": "~1.0",
         "sonata-project/exporter": "~1.0",
-        "sonata-project/block-bundle": "~2.3",
+        "sonata-project/block-bundle": "~2.3,>=2.3.7",
         "sonata-project/core-bundle": "^2.3.10",
         "doctrine/common": "~2.2",
         "doctrine/inflector": "~1.0",