Image assets no longer being published with gem

When we started using the gitlab.com/gitlab-org/components/gem-release/gem-release component, the gem started being built with gem build. Unfortunately this bypassed our build rake task which also built the image assets. So those were not being included in the gem.

Now we have overridden the script piece of the build-regular-gem job to run our rake task. We've also added a gem smoke test to ensure that an emoji can be found and its image file is available.

This can be tested in this MR by setting dry_run to true in the .gitlab-ci.yml file and prepending RELEASE to the MR title.

Closes #31 (closed)