errors with <visibility> def ... (ruby >= 2.1)

With the code below:
- method X::Y#foo is not documented
- class X::Z is documented as Z, with superclass Y (instead of X::Z < X::Y)

``` ruby
module X
  class Y
    protected def foo
    end
  end
  class Z < Y
    def foo
    end
  end
end
```
