在执行 pod install 后，得到一下错误：
[!] CDN: trunk Repo update failed - 4 error(s):
CDN: trunk URL couldn't be downloaded: https://cdn.jsdelivr.net/cocoa/Specs/c/7/9/Reachability/3.0.0/Reachability.podspec.json Response: Timeout was reached
CDN: trunk URL couldn't be downloaded: https://cdn.jsdelivr.net/cocoa/Specs/c/7/9/Reachability/3.1.0/Reachability.podspec.json Response: Timeout was reached
CDN: trunk URL couldn't be downloaded: https://cdn.jsdelivr.net/cocoa/Specs/c/7/9/Reachability/3.1.1/Reachability.podspec.json Response: Timeout was reached
CDN: trunk URL couldn't be downloaded: https://cdn.jsdelivr.net/cocoa/Specs/c/7/9/Reachability/3.2/Reachability.podspec.json Response: Timeout was reached
解决办法：
用source 'https://github.com/CocoaPods/Specs.git'替换到cdn源
步骤：
1.在podfile里面顶部添加source 'https://github.com/CocoaPods/Specs.git'
2.终端下执行 pod repo remove trunk来移除trunk
3.重新pod install
