Reinstalling ruby

I somehow screwed up my ruby installation. It says:

ruby ....
/Library/Ruby/Site/2.0.0/rubygems.rb:1098:in `register_default_spec': undefined method `default_gems_use_full_paths?' for Gem:Module (NoMethodError)
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/specification.rb:699:in `block in load_defaults'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/specification.rb:647:in `block (2 levels) in each_spec'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/specification.rb:643:in `each'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/specification.rb:643:in `block in each_spec'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/specification.rb:642:in `each'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/specification.rb:642:in `each_spec'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/specification.rb:653:in `each_default'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/specification.rb:698:in `load_defaults'
from /Library/Ruby/Site/2.0.0/rubygems.rb:1239:in `<top (required)>'
from <internal:gem_prelude>:1:in `require'
from <internal:gem_prelude>:1:in `<compiled>'

I updated all gems or something that like. How can I revert to the original ruby installation? I guess I have a timemachine backup, but maybe a clean reinstall is better.