Bump jemoji to v0.4.0

This release brings emoji to collection documents. The only important file is `lib/jemoji.rb`.

- Diff: https://github.com/jekyll/jemoji/compare/v0.3.0...v0.4.0
- Release: https://github.com/jekyll/jemoji/releases/tag/v0.4.0

/cc @gjtorikian, who wrote the beautiful code