added redis version req to readme
added redis version req to readme

fixes #1875

pana1990/yii2-redis,Saithiril/yii2-redis,E96/yii2-redis,cebe/jenkins-test,cebe/jenkins-test,yiijan/yii2-redis,liulingfree/yii2-redis,valinurovam/yii2-redis,yiisoft/yii2-redis