运行gem update --system时出现rubygems-update错误

Question

当我在MacOS Sierra中运行gem update --system时，我遇到了这样的问题：

Successfully installed rubygems-update-2.6.10
ERROR:  While executing gem ... (Errno::EACCES)
    Permission denied @ dir_s_mkdir - /usr/local/lib/ruby/gems/2.3.0/doc/rubygems-update-2.6.10