Add emoji_helper.rb instead of gitlab_emoji gem and upgrade Gemojione

What does this MR do?

• 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.

Changelog: https://github.com/jonathanwiesel/gemojione/blob/master/CHANGELOG.md

Are there points in the code the reviewer needs to double check?

That sprites and emoji aren't broken (they shouldn't be, I tested).

Why was this MR needed?

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

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together