给 Gem 换国内源
Replace the mirror source for Gem
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换成国内：
lets@eubt:~$ gem sources
*** CURRENT SOURCES ***
https://rubygems.org/
lets@eubt:~$ gem sources --remove https://rubygems.org/
https://rubygems.org/ removed from sources
lets@eubt:~$ gem sources --add https://mirrors.tuna.tsinghua.edu.cn/rubygems/
https://mirrors.tuna.tsinghua.edu.cn/rubygems/ added to sources
lets@eubt:~$ gem sources
*** CURRENT SOURCES ***
https://mirrors.tuna.tsinghua.edu.cn/rubygems/
lets@eubt:~$
更新缓存：
gem sources -u
