rvm gemset update
Running gem update for all rubies and gemsets.
Only 'do' action is allowed.
I’d never seen these before and was a bit perplexed. Luckily…
Simple cause, easy solution
If you’ve installed RVM, try this:
$ rvm -v
#=> rvm 1.23.4 (stable)...
$ rvm osx-ssl-certs status all
#=> Certificates for...
$ rvm osx-ssl-certs update all
#=> Updating certificates...
That’s all that is needed to resolve the issue if you are using RVM (you must be using RVM version 1.19.1 or newer).
For more on the issue, see a discussion at add rvm osx-ssl-certs on GitHub.
In a year or two, the error may reappear, when the certificate files again become outdated. To keep the certificate files current, RVM offers an option to run a cron job that updates the certificate files daily.
$ sudo rvm osx-ssl-certs cron install
Reposted in entirety from the excellent original article, which you should check out anyway. The link includes info for rbenv, macports, older rubies, and associated issues.