range组件回滚

### VUX version
2.9.4

### OS/Browsers version
iPhone 7P/Chrome 76

### Vue version
2.6.10

### Code

```html

import { Range } from 'vux'
// import { setInterval } from 'timers'
export default {
name: 'Percent',
components: {
Range
},
data() {
return {
percent: 0
}
},
mounted: function() {
let i = 115
setInterval(() => {
this.percent = Math.floor(i)
i = 0.32
}, 330)
}
}

.audio-percent {
width: 4.5rem
}

.audio-percent .range-bar {
background-color: hsla(0,0%,100%,.6);
height: .05rem!important
}

.audio-percent .range-bar .range-handle {
height: .3rem!important;
width: .3rem!important;
top: -.12rem!important;
-webkit-box-shadow: 0 0 0 .066667rem hsla(0,0%,100%,.6);
box-shadow: 0 0 0 .066667rem hsla(0,0%,100%,.6)
}

.audio-percent .range-bar .range-max,.audio-percent .range-bar .range-min {
display: none
}

.audio-percent .range-bar .range-quantity {
background-color: #fff
}
html {
font-size: 55.2px;
}

```

### Steps to reproduce
在i=200左右的时候，按钮会回滚。
可以打印源码中的components-range -utils.js的findClosest函数：
console.log(closest);
return closest;
可以看到。

### What is Expected?
返回的 closest 应该是递增的，不应该减小（除非拖动或重置）

### What is actually happening?
实际会出现 198 => 200 => 198，这样的行为