Fix missing !

Author: Justin Marney

lib/kafka/consumer_group.rb

@@ -48,7 +48,7 @@ def join
     rescue ConnectionError
       @logger.error "Connection error while trying to join group `#{@group_id}`; retrying..."
       sleep 1
-      @cluster.mark_as_stale
+      @cluster.mark_as_stale!
       @coordinator = nil
       retry
     end