[#493] Force loading the classes of activity provider

This will cause the class to run acts_as_journal which will create the
Journal sub-classes and prevent uninitialized constants since the sub-classes
are created on the fly.

db/migrate/20100714111652_update_journals_for_acts_as_journalized.rb

@@ -1,3 +1,5 @@
+Redmine::Activity.providers.values.flatten.uniq.collect(&:underscore).each {|klass| require_dependency klass }
+
 class UpdateJournalsForActsAsJournalized < ActiveRecord::Migration
   def self.up
     # This is provided here for migrating up after the JournalDetails has been removed