maintenance: prune approvers monthly instead of weekly.

* Move approver pruning from a weekly task to a monthly task.
* Move weekly maintenance tasks from Wednesdays at 1:30am to Sundays at
  midnight (UTC).

config/schedule.rb

@@ -3,14 +3,18 @@ env "RAILS_LOG_TO_STDOUT", "true"
 
 set :output, "log/whenever.log"
 
-every 1.hour do
+every :hour do
   rake "maintenance:hourly"
 end
 
-every 1.day do
+every :day do
   rake "maintenance:daily"
 end
 
-every 1.week, :at => "1:30 am" do
+every :sunday do
   rake "maintenance:weekly"
 end
+
+every :month do
+  rake "maintenance:monthly"
+end