delete certs that have long expired (6 months)

fixes #783

src/tasks.js

@@ -20,7 +20,7 @@ exports = module.exports = {
     TASK_APP: 'app',
     TASK_BACKUP: 'backup',
     TASK_UPDATE: 'update',
-    TASK_RENEW_CERTS: 'renewcerts',
+    TASK_CHECK_CERTS: 'checkCerts',
     TASK_SETUP_DNS_AND_CERT: 'setupDnsAndCert',
     TASK_CLEAN_BACKUPS: 'cleanBackups',
     TASK_SYNC_EXTERNAL_LDAP: 'syncExternalLdap',
@@ -39,7 +39,7 @@ exports = module.exports = {
     _TASK_SLEEP: '_sleep'
 };
 
-let assert = require('assert'),
+const assert = require('assert'),
     BoxError = require('./boxerror.js'),
     debug = require('debug')('box:tasks'),
     path = require('path'),