修改gem下載源

ERROR:  While executing gem ... (Gem::RemoteFetcher::FetchError)
    Errno::ECONNRESET: Connection reset by peer - SSL_connect (https://api.rubygems.org/quick/Marshal.4.8/bigdecimal-1.2.7.gemspec.rz)

$ gem sources --remove https://rubygems.org/
$ gem sources -a https://ruby.taobao.org/
$ gem sources -l
*** CURRENT SOURCES ***

https://ruby.taobao.org