解决The document “Main.storyboard”

问题：
This version does not support documents saved in the Xcode 9 format. Open this document with Xcode 9.0 or later.
原因：
用Xcode9打开的Xib，xcode会编译一遍。打上xcode9的印记～～～
解决：用文本编辑器打开保存的xib文件 手动改以下代码

    <dependencies>
        <plugIn identifier="com.apple.InterfaceBuilder.IBCocoaTouchPlugin" version="13104.12"/>
        <capability name="Safe area layout guides" minToolsVersion="9.0"/>
        <capability name="documents saved in the Xcode 8 format" minToolsVersion="8.0"/>
    </dependencies>

改为

 <dependencies>
        <plugIn identifier="com.apple.InterfaceBuilder.IBCocoaTouchPlugin" version="13104.12"/>
        <capability name="Safe area layout guides" minToolsVersion="8.0"/>
        <capability name="documents saved in the Xcode 8 format" minToolsVersion="8.0"/>
    </dependencies>