Fix wrong constraint for rollerworks/metadata

`The ^ operator behaves very similarly but it sticks closer to semantic versioning, and will always allow non-breaking updates.`
So `^0.4` disallows 0.5 (as 0.5 is considered breaking in terms of semver).

Version 0.5 is not really required, 0.4 will also work but 0.5 introduced the MappingDriverChain.

composer.json

@@ -17,7 +17,7 @@
     "require": {
         "php": ">=5.4",
         "rollerworks/search": "^1.0.0-beta7",
-        "rollerworks/metadata": "^0.4",
+        "rollerworks/metadata": ">=0.5,<2.0",
         "doctrine/annotations": "~1.1",
         "symfony/yaml": "~2.3"
     },