ERROR: While executing gem ... (Gem::DependencyError)
Unable to resolve dependencies: cocoapods requires cocoapods-core (= 1.1.1), cocoapods-downloader (< 2.0, >= 1.1.2), cocoapods-trunk (< 2.0, >= 1.1.1), xcodeproj (< 2.0, >= 1.3.3)
解决方法：sudo gem update --system
ERROR: While executing gem ... (Errno::EPERM)
Operation not permitted - /usr/bin/xcodeproj
解决方法：sudo nvram boot-args="rootless=0"; sudo reboot
重启后再执行安装即可。
sudo gem install -n /usr/local/bin cocoapods