Psych.safe_load relies on ClassLoader without explicit dependency

I'm seeing the following when invoking `safe_load` in certain places:

```
NameError: uninitialized constant Psych::ClassLoader
# /Users/ben/.rvm/gems/ruby-1.9.3-p448/gems/psych-2.0.2/lib/psych.rb:286:in `safe_load'
```