Steps for upgrading a Rails 3.x app to Ruby 2.2.0:
1. Install Ruby 2.2.0, e.g. brew update; brew upgrade ruby-build; rbenv install 2.2.0
2. Set app's ruby version to 2.2.0, e.g. rbenv local 2.2.0
3. Update gems that are broken, e.g. bundle update pg eventmachine rubocop
4. Add test-unit to Gemfile to fix "cannot load such file — test/unit/testcase (LoadError)" error:
```ruby
gem 'test-unit'
```
1. Rollback to 2.1 when you notice all warnings, related to e.g. Comparable, or continue with the warnings