首先MAC自带了ruby,他是用rvm 管理的， 如果你用homebrew又安装了一个新版，很可能在升级cocoapods时遇到问题，明明自己用homebrew升级到最新版本了，页配置环境变量了，为什么还是无法升级cocoapods?
因为系统默认还是用
要用到一个气泡显示器，用了git上面的一个第三方库
执行pod install 命令报错：
$ pod install
Analyzing dependencies
[!] Unable to satisfy the following requirements:
- `LFKit/Component/LFBubbleView` required by `Podfile`
None of your spec sources contain a spec satisfying the dependency: `LFKit/Component/LFBubbleView`.
You have either:
* out-of-date source repos which you can update with `pod repo update` or with `pod install --repo-update`.
* mistyped the name or version.
* not added the source repo that hosts the Podspec to your Podfile.
Note: as of CocoaPods 1.0, `pod repo update` does not happen on `pod install` by default.
然后，我按照提示输入命令：
$pod update --verbose
Update all pods
Preparing
Updating local specs repositories
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
后面还吐了一大堆，
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-core-1.3.1/lib/cocoapods-core/github.rb:105:in `rescue in modified_since_commit'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-core-1.3.1/lib/cocoapods-core/github.rb:100:in `modified_since_commit'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-core-1.3.1/lib/cocoapods-core/source.rb:445:in `unchanged_github_repo?'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-core-1.3.1/lib/cocoapods-core/source.rb:334:in `update'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-1.3.1/lib/cocoapods/sources_manager.rb:88:in `block (2 levels) in update'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-1.3.1/lib/cocoapods/user_interface.rb:64:in `section'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-1.3.1/lib/cocoapods/sources_manager.rb:87:in `block in update'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-1.3.1/lib/cocoapods/sources_manager.rb:86:in `each'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-1.3.1/lib/cocoapods/sources_manager.rb:86:in `update'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-1.3.1/lib/cocoapods/installer/analyzer.rb:237:in `block in update_repositories'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-1.3.1/lib/cocoapods/installer/analyzer.rb:235:in `each'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-1.3.1/lib/cocoapods/installer/analyzer.rb:235:in `update_repositories'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-1.3.1/lib/cocoapods/installer.rb:150:in `block in resolve_dependencies'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-1.3.1/lib/cocoapods/user_interface.rb:64:in `section'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-1.3.1/lib/cocoapods/installer.rb:149:in `resolve_dependencies'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-1.3.1/lib/cocoapods/installer.rb:116:in `install!'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-1.3.1/lib/cocoapods/command/update.rb:81:in `run'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/claide-1.0.2/lib/claide/command.rb:334:in `run'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-1.3.1/lib/cocoapods/command.rb:52:in `run'
/Users/username/.rvm/gems/ruby-2.4.1@global/gems/cocoapods-1.3.1/bin/pod:55:in `<top (required)>'
/Users/username/.rvm/rubies/ruby-2.4.1/bin/pod:23:in `load'
/Users/username/.rvm/rubies/ruby-2.4.1/bin/pod:23:in `<main>'
主要意思是说，要么你网络有问题，要么是GitHub挂了。
pod update 命令执行不成功，自然无法更新最新的 cocoapods 库索引了。
从提示信息上看，我的ruby 是 2.4.1， cocoapods 版本是1.3.1，算是比较新的了。
最后发现是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， 看GitHub的官方公告。
stackoverflow上已经有了解决方法：大致流程是：
$ 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
//这里可能 会跟你自己安装ruby的方式冲突，请看文章最后， 建议用 rvm 就用rvm, 用rbenv 就卸载rvm 再安装 rbenv，二选一
$ 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
但是我按照这些方法还是不行，原因可能是，我安装或升级pods时用的是 rvm管理的，再次升级安装，它会自动用用ruby 2.4.1 。而不会用brew安装的 2.5.1 。
使用命令：$ ruby --version
ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-darwin16]
发现确实是 bash 引用的还是 ruby 2.4.1
最后我觉得应该把 rvm 管理的系统ruby升级：
安装rvm
列出ruby可安装版本 rvm list known
安装一个需要的ruby版本 rvm install 2.5.0
$ brew upgrade openssl
但是在
rvm install 2.5.0 命令执行过程中发现， 他自动 升级了homebrew， 然后升级了 openssl到1.1 总之，这个 github加密升级的东西是搞定了。
Updating Homebrew...
==> Upgrading 3 outdated packages, with result:
coreutils 8.28_1 -> 8.29, openssl@1.1 1.1.0f -> 1.1.0h, automake 1.15.1 -> 1.16.1
==> Upgrading openssl@1.1
==> Downloading https://homebrew.bintray.com/bottles/openssl@1.1-1.1.0h.high_sie
######################################################################## 100.0%
==> Pouring openssl@1.1-1.1.0h.high_sierra.bottle.tar.gz
CocoaPods 1.5.0 is available.
To update use: `sudo gem install cocoapods`
For more information, see https://blog.cocoapods.org and the CHANGELOG for this version at https://github.com/CocoaPods/CocoaPods/releases/tag/1.5.0
Pod installation complete! There are 11 dependencies from the Podfile and 11 total pods installed.
关于ruby 多版本的问题以后再做探讨，似乎有一个管理多版本ruby的第三方库。