I ran into a problem with the Ruby gem for a tool I use recently when I installed the latest version and then found it was broken.  I wanted to roll back to the previous version, which turns out to be extremely simple.  Here is the command I used and the responses I received:

> sudo gem uninstall cocoapods
Password:

Select gem to uninstall:
1. cocoapods-0.23.0
2. cocoapods-0.24.0
3. cocoapods-0.25.0
4. cocoapods-0.26.2
5. cocoapods-0.27.1
6. cocoapods-0.28.0
7. cocoapods-0.29.0
8. cocoapods-0.30.0
9. All versions
>

I selected 8 in the list, and when I checked the version of the gem after running this command, it had indeed been rolled back.  Simple.  Sweet.  Just what I needed.