Fix pod update issue after change the admin user of the mac

After changing the own of the mac, there may be errors when updating pod in the existing xcode project. The main reason is the brew, gem, ruby, cocoapod fail to upgrade due the permission issue.

The below are steps to update the mac

1.  give permission to install homebrew
sudo chmod -R 777 /usr/local/Homebrew

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

3. Install ruby
brew install ruby

4. update gem
gem update --system

5. install cocoapods
sudo gem install cocoapods -n/usr/local/bin

6. go to your xcode project to update pod dependent libraries
pod update 

The above steps also applies to the refresh mac pod initialization.