Add and apply Style/RedundantBegin rubocop rule

We're about to add code which might fall into the `RedundantBegin`
category, so we're adding the rule in order to prevent that.

spec/models/poll/answer_spec.rb

@@ -187,10 +187,8 @@ describe Poll::Answer do
 
       [answer, other_answer].map do |poll_answer|
         Thread.new do
-          begin
-            poll_answer.save_and_record_voter_participation
-          rescue ActiveRecord::RecordInvalid
-          end
+          poll_answer.save_and_record_voter_participation
+        rescue ActiveRecord::RecordInvalid
         end
       end.each(&:join)