Make the backup cronjob run every 4 hours again

This only calls ensureBackup() which in turn would only
trigger one once per day

src/cron.js

@@ -66,7 +66,7 @@ function recreateJobs(unusedTimeZone, callback) {
 
         if (gBackupJob) gBackupJob.stop();
         gBackupJob = new CronJob({
-            cronTime: '00 00 00 * * *', // every day once at midnight
+            cronTime: '00 00 */4 * * *', // every 4 hours. backups.ensureBackup() will only trigger a backup once per day
             onTick: backups.ensureBackup.bind(null, AUDIT_SOURCE, NOOP_CALLBACK),
             start: true,
             timeZone: allSettings[settings.TIME_ZONE_KEY]