upgrade(appveyor): test under node v8
upgrade(appveyor): test under node v8

zixia/wechaty,binsee/wechaty,mukaiu/wechaty,Chatie/wechaty,mukaiu/wechaty,Chatie/wechaty,zixia/wechaty,wechaty/wechaty,mukaiu/wechaty,lijiarui/wechaty,binsee/wechaty,wechaty/wechaty,Chatie/wechaty,lijiarui/wechaty,binsee/wechaty,wechaty/wechaty