Increase poll stats performance

Using SQL's `select` instead of converting the records to a ruby array
increases performance dramatically when there are thousands of records.
For a poll with 200000 voters, calculating stats took more than 7
minutes, and now it takes less than 2 minutes.

app/models/poll/stats.rb

@@ -92,11 +92,11 @@ class Poll::Stats
   private
 
     def participant_ids
-      voters.pluck(:user_id)
+      voters
     end
 
     def voters
-      @voters ||= poll.voters
+      @voters ||= poll.voters.select(:user_id)
     end
 
     def recounts