Bump mentions to v0.0.7

https://github.com/jekyll/jekyll-mentions/releases/tag/v0.0.7
Bump to jekyll-mentions 0.0.8
Ruby doesn't speak English

https://github.com/jekyll/jekyll-mentions/commit/d5ff484f8ed4d6aeabc492255aaaf68063bafda8
Merge pull request #70 from github/mentions-0.0.7

Bump mentions to v0.0.9