ruby 无法安装gem包及添加gem国内源

[kematy]

[admin@TNAS-qin desktop]$ gem source -a https://gems.ruby-china.com
Failed to load /home/admin/.gemrc, (<unknown>): could not find expected ':' while scanning a simple key at line 7 column 1
ERROR: SSL verification error at depth 1: unable to get local issuer certificate (20)
ERROR: You must add /O=Digital Signature Trust Co./CN=DST Root CA X3 to your local trusted store
Error fetching https://gems.ruby-china.com:
SSL_connect returned=1 errno=0 state=error: certificate verify failed (https://gems.ruby-china.com/specs.4.8.gz)
请问如何认证该网站；
gem install ffi
Fetching: ffi-1.13.1.gem (100%)
ERROR: While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /usr/local/lib/ruby/gems/2.4.0 directory.
没有写入的权限。我使用命令也不允许更改文件夹权限。
如果不能安装ruby的gems，ruby就没有什么用了。
请帮忙搞定。

[TMzethar]

收到您的反馈了，我们会参考你给出的信息验证下下该问题的。

[TMzethar]

1. 先删除原gem源：gem sources -r https://rubygems.org/
2. 添加国内源：gem sources -a https://gems.ruby-china.com
3. 看看是否成功：gem sources -l