更新gem发现老是报错，刚开始没想到是gem源的问题，后面检查发现是gem源的问题，错误提示如下：
zoujies-MacBook-Air:~ zoujie$ gem sources -a https://ruby.taobao.org/
ERROR: SSL verification error at depth 0: ok (0)
Error fetching https://ruby.taobao.org/:
SSL_connect returned=1 errno=0 state=SSLv3 read server certificate B: certificate verify failed (certificate rejected) (https://ruby.taobao.org/specs.4.8.gz)
zoujies-MacBook-Air:~ zoujie $ gem sources -a http://gems.ruby-china.org/
Error fetching http://gems.ruby-china.org/:
bad response Not Found 404 (http://gems.ruby-china.org/specs.4.8.gz)
zoujies-MacBook-Air:~ zoujie$ gem sources -a https://gems.ruby-china.org/
Error fetching https://gems.ruby-china.org/:
bad response Not Found 404 (https://gems.ruby-china.org/specs.4.8.gz)
最后发现，正确的gem源是：gem sources -a https://gems.ruby-china.com/
