Allow for legacy migration to generate burndown data

lib/tasks/generate_chart_data.rake

@@ -3,7 +3,7 @@ desc 'Generate chart data for all backlogs'
 namespace :redmine do
     namespace :backlogs_plugin do
         task :generate_chart_data => :environment do
-            Sprint.generate_burndown
+            Sprint.generate_burndown(!ENV['all'])
         end
     end
 end