Fix Performance/HashEachMethods offenses

values.each and keys.each methods are depreciated, so they have replaced by
each_value and each_key respectively. This has done manually.

Closes https://github.com/openSUSE/osem/issues/1825

db/migrate/20140701123203_add_events_per_week_to_conference.rb

@@ -77,7 +77,7 @@ class AddEventsPerWeekToConference < ActiveRecord::Migration
 
       hash.each do |week, values|
         if previous
-          values.each do |state, _value|
+          values.each_key do |state|
             hash[week][state] += previous[state]
           end
         end