Add Ruby gem source code release to CI/CD pipeline

Right now, only platform-specific precompiled gems are uploaded to RubyGems.

This means that several platforms are not supported via RubyGems. Furthermore, some tools expect there to always be a generic gem release. For instance, RubyGems itself shows 0.11.2 as the current release, which is not correct. That was the last source release.

Therefore I would like to suggest to add a source code release to the GitHub workflow.