Set Redis retry strategy for emergency banner

This brings the Redis connection retry strategy in line with the
configuration specified in `govuk_sidekiq` [1].

1: https://github.com/alphagov/govuk_sidekiq/blob/32def77d86d929ca7eb48f35f08401fedeb8407d/lib/govuk_sidekiq/sidekiq_initializer.rb#L10-L12

lib/emergency_banner/display.rb

@@ -2,7 +2,11 @@ module EmergencyBanner
   class Display
     class << self
       def client
-        @client ||= Redis.new(timeout: 0.1)
+        @client ||= Redis.new(
+          reconnect_attempts: 4,
+          reconnect_delay: 15,
+          reconnect_delay_max: 60,
+        )
       end
     end