Silence Puma in system tests

See rspec/rspec-rails#1897 - this should be
silenced already but something in Rails 5.2 causes it not to be.

Without this we see the Puma startup message in our spec output.

spec/support/capybara.rb

@@ -7,3 +7,4 @@
 end
 
 Capybara.javascript_driver = :headless_chrome
+Capybara.server = :puma, { Silent: true }