message: fix(observe-image): convert webp to png because of compatity under iOS 13
diff: <script type="text/ecmascript-6">
  import BScroll from '@better-scroll/core'
  import ObserveImage from '@better-scroll/observe-image'
  - import carbinImage from './cabin.webp'
  - import homeImage from './home.webp'
  - import stoneImage from './stone.webp'
  - import zooImage from './zoo.webp'
  BScroll.use(ObserveImage)
  export default {
  data () {
  return {
  - images: [carbinImage, homeImage, stoneImage, zooImage]
  + images: [
  + 'https://dpubstatic.udache.com/static/dpubimg/dEswI1MVy6/zoo.png',
  + 'https://dpubstatic.udache.com/static/dpubimg/BYb_wPak21/home.png',
  + 'https://dpubstatic.udache.com/static/dpubimg/B6q1pWB0sB/cabin.png',
  + 'https://dpubstatic.udache.com/static/dpubimg/76n1ilzf4R/stone.png'
  + ]
  }
  },
  mounted() {
  