gem install could not find error

When doing gem insall <pkg>, ex., gem install sass,
if you get the error like this:

Could not find a valid gem 'sass' (>=0), here is why:
Unable to download data from https://rubygems.org/ - SSL_connect returned=1 errno=0 state=SSLv3 read server certificate B: certificate verify failed(https://api.rubygems.org/latest_specs.4.8.gz)

You can solve the problem by typing the command:

gem source -a http://rubygems.org/

And then do the gem install sass again.