I was trying to
gem update but it was saying 'everything is up-to-date'. Nothing was happening. I could tell from rubygems.org that there were new versions available, and even a new version of rubygems itself. Here's what I had to do:
$ wget http://production.cf.rubygems.org/rubygems/rubygems-update-2.1.8.gem
$ gem install rubygems-update-2.1.8.gem
$ gem --version
Hat tip to trevor here: https://github.com/rubygems/rubygems/issues/515
Note: if you are using RVM, this may be easier.