gem-maven-plugin uses the deprecated "--no-rdoc" command

When running the initialize command, I get the following error message

[WARNING] ERROR:  While executing gem ... (OptionParser::InvalidOption)
    invalid option: --no-rdoc```

According to https://github.com/rubygems/heroku-buildpack-bundler2/pull/1#issuecomment-451654992, it seems like the `--no-rdoc` flag is deprecated and should be replaced by `--no-document`.