The default Ruby version installed in the OS X containers is 2.0.0p648. Sometimes you would want to use a newer Ruby in your build—in this case you can get the newest Ruby available through Homebrew installed with the following content of your circle.yml:
machine:
pre:
- brew install ruby ; brew link --overwrite ruby
After choosing a different Ruby version please make sure to do a rebuild without cache so that all the gems are rebuilt with the new version of the language.