Add lib/gitlab/emoji.rb instead of gitlab_emoji gem and upgrade Gemojione

- No reason to split it into a separate gem when the gem barely did anything. We can use gemojione directly, making updating gemojione that much easier. Unless there's a particularly good reason we were using the gem?
- Fixes the Rake task since it broke after all the AwardEmoji changes.
- Update gemojione to 2.6.1.

Spring Update changes! http://emojione.com/releases/#2.2.0

See merge request !4919