Update repo to use prawn-dev

[petergoldstein]

The prawn-dev update sets the minimum Ruby version to 2.6.

In addition this PR:

• Sets up Rubocop with a todo file that can be burned down.
• Adds a code-style lint action matching prawn
• Removes Ruby 2.5, 2.5.0, and jruby-9.2 from the CI matrix
• Fixes a few lints