今天pod search 一个库时发现搜索到的版本比github上的最新版本旧，于是采用下面的命令更新Cocoapods的库
pod repo update master --verbose
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
:
2.1.0
2.1.1
2.1.2
:
$ rbenv install 2.1.0
$ rbenv versions
* system (set by /Users/username/.rbenv/version)
2.1.0
$ ruby --version
ruby 2.0.0p648 (2015-12-16 revision 53162) [universal.x86_64-darwin16]
$ rbenv global 2.1.0
$ rbenv versions
system
* 2.1.0 (set by /Users/username/.rbenv/version)
$ ruby --version
ruby 2.1.0p0 (2013-12-25 revision 44422) [x86_64-darwin16.0]
$ 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。
