prepare for 2.0.0-alpha release.

composer.json

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