The error has something to do with being vulnerable to the Poodle SSL bug, it will not be verified for that reason. If there’s a way to upgrade to a better certificate, but at the time of writing this answer, I could not find the upgraded certificate.
I used the non-SSL host instead, altough I should note that this is not the best nor a permanent solution, it lacks security.
The command used:
gem source -a http://rubygems.org/
A discussion about this subject can be found here: https://github.com/rubygems/rubygems/issues/515#issuecomment-65326585
Update: There seems to be a permanent solution now, which replaces the certificate with a proper protected one. It can be found on the following URL, an tutorial is included in that page.
https://gist.github.com/luislavena/f064211759ee0f806c88#installing-using-update-packages-new