Improve dependencies

* Core bundle is now stable
* Use ^3.0 notation even for dev ones

composer.json

@@ -36,8 +36,8 @@
         "twig/twig": "^1.23",
         "twig/extensions": "^1.0",
         "sonata-project/exporter": "^1.0",
-        "sonata-project/block-bundle": "3.x-dev@dev",
-        "sonata-project/core-bundle": "3.x-dev@dev",
+        "sonata-project/block-bundle": "^3.0@dev",
+        "sonata-project/core-bundle": "^3.0",
         "doctrine/common": "^2.2",
         "doctrine/inflector": "^1.0",
         "knplabs/knp-menu-bundle": "^2.1.1"