pod update -- Failed to connect to GitHub to update the CocoaPods/Specs specs repo 问题

执行后却报了下面的错误：

Updating spec repo `master`
[!] Failed to connect to GitHub to update the CocoaPods/Specs specs repo - Please check if you are offline, or that GitHub is down

这是为何呢，明明不久前刚刚更新Cocoapods为1.3.1版本啊。
搜索才发现是Github在不久之前的2018年2月23号移除了一些低加密标准协议，包括TLSv1/TLSv1.1，diffie-hellman-group1-sha1，diffie-hellman-group14-sha1，相关链接在这里：Weak cryptographic standards removed
这就意味着Mac系统对应的openssl也需要更新。
这里提供了全套升级办法，搬运过来：

$ which openssl
/usr/bin/openssl

$ openssl version
OpenSSL 0.9.8zh 14 Jan 2016

$ /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)" 
$ brew update

$ brew install openssl

$ brew upgrade openssl

`` If you need to have this software first in your PATH run: echo 'export PATH="/usr/local/opt/openssl/bin:$PATH"' >> ~/.bash_profile

$ echo 'export PATH="/usr/local/opt/openssl/bin:$PATH"' >> ~/.bash_profile
$ source ~/.bash_profile

$ which openssl
/usr/local/opt/openssl/bin/openssl

$ openssl version
OpenSSL 1.0.2n  7 Dec 2017

$ brew install rbenv ruby-build

$ echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.bash_profile
$ echo 'if which rbenv > /dev/null; then eval "$(rbenv init -)"; fi' >> ~/.bash_profile
$ source ~/.bash_profile

$ rbenv install --list

Available versions:
1.8.5-p52
1.8.5-p113
1.8.5-p114
1.8.5-p115
1.8.5-p231
1.8.6
:
2.5.0-rc1
2.5.0
2.6.0-dev
:

$ rbenv install 2.5.0

$ rbenv versions
* system (set by /Users/username/.rbenv/version)
2.5.0

$ ruby --version
ruby 2.0.0p648 (2015-12-16 revision 53162) [universal.x86_64-darwin16]

$ rbenv global 2.5.0

$ rbenv versions
system
* 2.5.0 (set by /Users/username/.rbenv/version)

$ ruby --version
ruby 2.5.0p0 (2017-12-25 revision 61468) [x86_64-darwin16]

$ gem install cocoapods -n /usr/local/bin

$ which pod
/usr/local/bin/pod

$ pod --version
1.4.0

完成上面一系列操作后，Cocoapods将会升级到最新的1.4.0版本。再执行

pod repo update master --verbose

就没有问题了。
链接：https://www.jianshu.com/p/a502eb72b689