Add linux cron option to settings

[MAILPOET-1538]

lib/Cron/CronTrigger.php

@@ -10,6 +10,7 @@ class CronTrigger {
   public static $available_methods = array(
     'mailpoet' => 'MailPoet',
     'wordpress' => 'WordPress',
+    'linux_cron' => 'Linux Cron',
     'none' => 'Disabled'
   );
   const DEFAULT_METHOD = 'WordPress';
@@ -37,4 +38,4 @@ class CronTrigger {
   static function getCurrentMethod() {
     return Setting::getValue(self::SETTING_NAME . '.method');
   }
-}
+}