Incompatible with Ruby 2.6

[dayyan]

After adding DRB, this uses ruby2_keywords which isn't available in 2.6

[briandunn]

Here are the supported ruby versions
https://github.com/briandunn/flatware/blob/master/.github/workflows/ruby.yml#L23

If you'd like to restore support for 2.6 there should be prior versions to guide you.