Support Ruby 3

[calleluks]

Ruby 3 changes how keyword arguments work by separating them from positional arguments. Running on Ruby 3 requires explicitly delegating keyword arguments. By using the ruby2_keywords gem we can maintain backwards compatibility with Ruby versions prior to 2.7.

More info: https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/

This PR also upgrades mimemagic since the previously used version was yanked due to licensing concerns.