Bump Jekyll to 2.0.0

It's out! :sparkling_heart: 

- [x] Bump Jekyll to 2.0.3 (https://github.com/jekyll/jekyll/compare/v1.5.1...v2.0.3)
- [x] Bump redcarpet to 3.1.1 (https://github.com/vmg/redcarpet/compare/v2.3.0...v3.1.1)
- [x] Bump jekyll-redirect-from to 0.4.0 (https://github.com/jekyll/jekyll-redirect-from/releases/tag/v0.4.0)
- [x] Bump jemoji to 0.2.0 (https://github.com/jekyll/jemoji/releases/tag/v0.2.0)
- [x] Bump jekyll-sitemap to 0.4.0 (https://github.com/jekyll/jekyll-sitemap/releases/tag/v0.4.0, https://github.com/jekyll/jekyll-sitemap/releases/tag/v0.3.0)
- [x] Bump jekyll-mentions to 0.1.0 (https://github.com/jekyll/jekyll-mentions/releases/tag/v0.1.0)
- [x] Add CoffeeScript converter dependency and lock to 1.0.0
- [x] Add Sass converter dependency and lock to 1.0.0

http://jekyllrb.com/news/2014/05/06/jekyll-turns-2-0-0/