www/ruby-unicorn: update to 5.7.0

Update ruby-unicorn package to 5.7.0.

5.7.0 (2020-09-08)

Changes:

    unicorn 5.7.0

    Relaxed Ruby version requirements for Ruby 3.0.0dev.
    Thanks to Jean Boussier for testing

5.6.0 (2020-07-26)

Changes:

    unicorn 5.6.0 - early_hints support

    This release adds support for the early_hints configurator
    directive for the 'rack.early_hints' API used by Rails 5.2+.

    Thanks to Jean Boussier for the patch.

(taca)