Composer: prefer stable, but allow dev releases

composer.json

 {
   "name": "t3o/typo3.org",
   "description": "Website for TYPO3.org",
-  "minimum-stability": "stable",
+  "minimum-stability": "dev",
+  "prefer-stable": true,
   "authors": [
     {
       "name": "t3o team",