This is the name of the rubygem. Example. jenkins_api_client
The plugin will search for rubygems in the root of the workspace directory.
If multiple files are found matching the gem name in working directory, the
file with the gemfile that is recently modified will be chosen to publish the
gem.
If the name and version is known, it complete name can be specified.
Example: jenkins_api_client-0.13.0.gem