prepare for 2.0.0-rc release.

composer.json

@@ -12,7 +12,7 @@
         "irc": "irc://irc.freenode.net/yii",
         "source": "https://github.com/yiisoft/yii2"
     },
-    "minimum-stability": "dev",
+    "minimum-stability": "RC",
     "require": {
         "php": ">=5.4.0",
         "yiisoft/yii2": "*",