Check isActive when Redis server down

[oyajiro]

If i use Yii::$app->redis->open(); when redis server down. I want to switch to alternative DB. So i use Yii::$app->redis->open(); in try catch structure. After it, if i check on Yii::$app->redis->isActive it's true, even if redis not working(or not installed)

[samdark]

#6

[cebe]

fixed by #106