Add table to store stats versions

We need a way to manually expire the cache for a budget or poll without
expiring the cache of every budget or poll.

Using the `updated_at` column would be dangerous because most of the
times we update a budget or a poll, we don't need to regenerate their
stats.

We've considered adding a `stats_updated_at` column to each of these
tables. However, in that case we would also need to add a similar column
in the future to every process type whose stats we want to generate.

app/models/poll/stats.rb

@@ -107,7 +107,7 @@ class Poll::Stats
     stats_cache :participants, :voters, :recounts
 
     def stats_cache(key, &block)
-      Rails.cache.fetch("polls_stats/#{poll.id}/#{key}/v12", &block)
+      Rails.cache.fetch("polls_stats/#{poll.id}/#{key}/#{version}", &block)
     end
 
 end