I don't like that Bundler now makes Gemfile.lock change whenever I bump
up gemoji version. There aren't many dependencies, so a missing
Gemfile.lock won't slow down CI too much.
Gemoji implementation depends on `codepoints` and test depend on some
more Ruby 1.9-only features, so don't pretend that the library will be
1.8 compatible anytime soon. Probably never.
This reverts commit 6a781b6474.