https://gems.ruby-china.org/
修改镜像地址过程请注意，如果提示
Error fetching https://gems.ruby-china.org/:
SSL_connect returned=1 errno=0 state=SSLv3 read server certificate B:
certificate verify failed (https://gems.ruby-china.org/specs.4.8.gz)
请将地址中的 https 改为 http，重新尝试