jemoji 0.3.0

Looks like the strict dependency on HTML pipeline in at least one other gem is hampering this upgrade. I'll investigate. Probably mentions.

Depends on merge of #75.

Upgraded:

- `jemoji` v0.2.0 to v0.3.0 (https://github.com/jekyll/jemoji/compare/v0.2.0...v0.3.0)
- `gemoji` v1.5.0 to v2.0.0 (https://github.com/github/gemoji/compare/v1.5.0...v2.0.0)
- `html-pipeline` v1.5.0 to v1.9.0 (https://github.com/jch/html-pipeline/compare/v1.5.0...v1.9.0)