Add rack as a dependency

[pjmartorell]

Add rack (https://github.com/rack/rack) as a dependency, since I'm trying to run a very simple proxy app with puma (bundle exec puma -C proxy.ru) with rack-forward as a middleware and it's crashing:

bundler: failed to load command: puma (/Users/xxxxx/.rbenv/versions/2.6.5/bin/puma)
LoadError: cannot load such file -- rack/utils
  /Users/xxxxx/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/bundler/gems/rack-forward-0bc90c50126a/lib/rack/forward.rb:2:in `require'
  /Users/xxxxx/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/bundler/gems/rack-forward-0bc90c50126a/lib/rack/forward.rb:2:in `<top (required)>'
  proxy.ru:1:in `require'
  proxy.ru:1:in `_load_from'
  /Users/xxxxx/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-3.12.0/lib/puma/dsl.rb:41:in `instance_eval'
  /Users/xxxxx/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-3.12.0/lib/puma/dsl.rb:41:in `_load_from'
  /Users/xxxxx/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-3.12.0/lib/puma/configuration.rb:192:in `block in load'
  /Users/xxxxx/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-3.12.0/lib/puma/configuration.rb:192:in `each'
  /Users/xxxxx/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-3.12.0/lib/puma/configuration.rb:192:in `load'
  /Users/xxxxx/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-3.12.0/lib/puma/launcher.rb:59:in `initialize'
  /Users/xxxxx/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-3.12.0/lib/puma/cli.rb:69:in `new'
  /Users/xxxxx/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-3.12.0/lib/puma/cli.rb:69:in `initialize'
  /Users/xxxxx/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-3.12.0/bin/puma:8:in `new'
  /Users/xxxxx/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-3.12.0/bin/puma:8:in `<top (required)>'
  /Users/xxxxx/.rbenv/versions/2.6.5/bin/puma:23:in `load'
  /Users/xxxxx/.rbenv/versions/2.6.5/bin/puma:23:in `<top (required)>'