Draft: Replace gem source to RubyChina

Background

https://gems.ruby-china.com/ is a mirror from https://rubygems.org. it is completely implemented based on CDN, which can ensure synchronization with almost no delay.

See more: https://gems.ruby-china.com/

What does this MR do

Replace the source of jh/Gemfile from https://rubygems.org to https://gems.ruby-china.com.

This will speed up the download speed in the China mainland, and will save international traffic.