github-pages `jekyll-sass-converter` dependency is still version `1.5.2`

### Before submitting an issue, please be sure to

- [x] Read [the contributing instructions](https://github.com/github/pages-gem/blob/master/docs/CONTRIBUTING.md#contributing-to-the-github-pages-gem)
- [x] Update to the latest Gem version (run `bundle update github-pages`)

### This issue affects

- [x] The site generated by GitHub Pages
- [x] Building sites locally

### Questions
Currently in the dependencies file `jekyll-sass-converter` is still version `1.5.2` but the current version available is `2.1.0`, my question is why **[hubot](https://github.com/github/pages-gem/commits?author=hubot)** haven't bump to the new version ?
Is there dependencies issues ? Where can I find theses info ?

### Additional information
Link to the release page of `jekyll-sass-converter`:
https://github.com/jekyll/jekyll-sass-converter/releases

Github Pages dependencies file:
https://github.com/github/pages-gem/blob/master/lib/github-pages/dependencies.rb#L11