Could not find gem 'rogue (>= 0) ruby'

I add these lines to my Gemfile:

```
gem 'rogue', :github => "logdown/rouge", :branch => "wrapper_tag"
gem 'ultra_markdown'
```

and type command 'bundle'
An error has appear

```
Could not find gem 'rogue (>= 0) ruby' in git://github.com/logdown/rouge.git (at wrapper_tag).
Source does not contain any versions of 'rogue (>= 0) ruby'
```
