问题描述：想更新CocoaPods，在终端运行sudo gem update cocoapods，出现如下错误：
Updating cocoapods
Building native extensions. This could take a while...
ERROR: Error installing cocoapods:
ERROR: Failed to build gem native extension.
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb
mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/ruby.h
Gem files will remain installed in /Library/Ruby/Gems/1.8/gems/xcodeproj-0.13.1 for inspection.
Results logged to /Library/Ruby/Gems/1.8/gems/xcodeproj-0.13.1/ext/xcodeproj/gem_make.out
问题解决：Go to Xcode, Preferences -> Downloads，download Command Line Tools.