Expire budget stats cache when a phase is finished

This way we recalculate all data including the participants in the phase
which has just finished.

app/models/budget/stats.rb

@@ -178,6 +178,6 @@ class Budget::Stats
     stats_cache :voters, :participants, :authors, :balloters, :poll_ballot_voters
 
     def stats_cache(key, &block)
-      Rails.cache.fetch("budgets_stats/#{budget.id}/#{key}/v10", &block)
+      Rails.cache.fetch("budgets_stats/#{budget.id}/#{phases.join}/#{key}/v10", &block)
     end
 end