当我尝试更新宝石时，我得到了这个：
LJ$ gem update --system
ERROR: While executing gem ... (Gem::RemoteFetcher::UnknownHostError)
no such name
这时尝试安装gem：
LJ$ gem install mongrel
ERROR: Could not find a valid gem 'mongrel' (>= 0), here is why:
Unable to download data from https://rubygems.org/ - no such name
问题是什么？