slack-messenger points to a missing homepage

Summary

Also it is missing the license text in the gem.

Originally found in debian https://alioth-lists.debian.net/pipermail/pkg-ruby-extras-maintainers/2020-April/053400.html

I am opening the issue here as I can see all owners are from gitlab. @rspeicher @stanhu @justin_ho @rymai ...

I assume this is a fork by gitlab.

Possible fixes

Push the code to gitlab.com/gitlab-org namespace like other forks, update the homepage field in gemspec file, add full text of the license (possibly from the original code).