Update composer.json

Author: takuya

composer.json

@@ -37,7 +37,7 @@
         "preferred-install": "dist",
         "sort-packages": true
     },
-    "minimum-stability": "dev",
+    "minimum-stability": "stable",
     "prefer-stable": true,
     "repositories": {
         "php-nulab-backlog-api-client": {