Comment 20 for bug 1310292

I ran into this today. I was surprised that the update-alternatives support for ruby had been yanked. Here's how to set ruby2.0 to be the default:

sudo ln -sf /usr/bin/ruby2.0 /usr/bin/ruby
sudo ln -sf /usr/bin/gem2.0 /usr/bin/gem

No promises this will work 100% or not cause problems with other installed ruby dependent software.

ruby -v

should produce:
ruby 2.0.0p384 (2014-01-12) [x86_64-linux-gnu]

gem -v

should produce:
1.8.23