pod update出现：
[!] Unable to add a source with url https://github.com/CocoaPods/Specs.git named master-1.
You can try adding it manually in ~/.cocoapods/repos or via pod repo add.
这样的提示。
解决办法：由于装了多个Xcode，在终端输入：sudo xcode-select -switch /Applications/Xcode.app
