ruby gem update和gem update --system error

Question

在我的Linux系统Ubuntu 14.04.2上，当我运行gem update时，我得到了这个：

Updating installed gems
ERROR:  While executing gem ... (Gem::RemoteFetcher::FetchError)
Errno::ECONNREFUSED: Connection refused - connect(2) for "s3.amazonaws.com" port 443 (https://api.rubygems.org/specs.4.8.gz)

当我运行gem update --system时，我得到了这个：

Updating rubygems-update
ERROR:  While executing gem ... (Gem::SpecificGemNotFoundException)
Could not find a valid gem 'rubygems-update' (= 2.4.8) locally or in a repository

如何解决此问题？