Rdoc preview: uninitialized constant GitHub::Markup::RDoc 

When I try to use vim-preview with Rdoc I catch such error:

```
Обнаружена ошибка при обработке function <SNR>80_Preview..preview#show:
строка    4:
NameError: (eval):74:in `show_rdoc': uninitialized constant GitHub::Markup::RDoc 
```

I have github-markup gem installed both on system and on default rvm ruby. System Ruby is 1.8.7
