Run feature specs using Puma

Webrick doesn't support Rack hijack and we need it for ActionCable

Also, we now use Puma in production so it's also good to use it in
our feature specs

spec/support/capybara.rb

@@ -60,7 +60,6 @@ Capybara.register_driver :chrome do |app|
   )
 end
 
-Capybara.server = :webrick
 Capybara.javascript_driver = :chrome
 Capybara.default_max_wait_time = timeout
 Capybara.ignore_hidden_elements = true