1. gem install redis

https://rubygems.org/gems/redis/versions/3.3.5

    pwd
    ls /home/jlch/
    pwd
wget https://rubygems.org/downloads/redis-3.3.5.gem
    gem install /home/jlch/redis-4.0.1.gem  ## ruby-2.4 会成功，其它报错
    gem install /home/jlch/redis-4.0.0.gem  ## 报错
    gem install /home/jlch/redis-3.3.5.gem  ## 成功