Support Ruby 2.0

```
[1] pry(main)> class C; end
=> nil
[2] pry(main)> module M; end
=> nil
[3] pry(main)> C.prepend M
NoMethodError: private method `prepend' called for C:Class
from (pry):3:in `__pry__'
[4] pry(main)> RUBY_VERSION
=> "2.0.0"
```