[Bug Report] popup组件引入后报错，找不到vux-modal.css

### VUX version
2.9.2

### OS/Browsers version
chrome 71.0.3578.98

### Vue version
2.5.21

### Code

```html
//in ./node_modules/vux/src/components/popup/index.vue
Module build failed: Error: Failed to find '../../styles/vux-modal.css'

@import '../../styles/vux-modal.css';
```

### Steps to reproduce
按照官方文档引入Popup组件，运行时报错找不到vux-modal.css，但是包里面是包含这个文件，路径也是正确的

### What is Expected?
能正常使用Popup组件

### What is actually happening?
Popup组件引用后不能使用