Cocoapods 1.8.5执行pod update或者pod install时报错如下：
Update all pods
Updating local specs repositories
[!] CDN: trunk URL couldn't be downloaded: https://raw.githubusercontent.com/CocoaPods/Specs/master/Specs/ Response: 400
解决办法：
按照官方文档 podfile文件中添加source源:
# Uncomment the next line to define a global platform for your project
source 'https://github.com/CocoaPods/Specs.git'
platform :ios, '10.0'
... ...
... ...