For some reason, rbenv-default-gems didn't run at some point, and so my gems aren't installed. I tried to install them by running the install script by itself. Here's the output in zsh:

☰  bash install/default-gems.bash
rbenv: rbenv-default-gems plugin requires ruby-build 20130129 or later

I've got the latest Rbenv and Ruby-build repos. I'm not sure if the script was intended to be used this way, but it'd be nice to have just in case.