How do you downgrade rubygems?
Solution 1:
This worked for me when downgrading from 1.5.2 to 1.4.2 because of old rails version:
sudo gem update --system 1.4.2
More information about downgrading/upgrading rubygems: https://github.com/rubygems/rubygems/blob/master/UPGRADING.md
Solution 2:
Updated November 2011: Now that RVM's out, try running rvm install rubygems <version>.
Solution 3:
If you're using RVM, then you can do this to remove current ruby gems, then upgrade/downgrade rubygems:
rvm install rubygems 1.4.2