Improves Code Climate rating on the Notification class

Currently the `Gomoob\Pushwoosh\Model\Notification\Notification` class has an F note on Code Climate. This is bad and we have to fix it. 

See https://codeclimate.com/github/gomoob/php-pushwoosh/src/main/php/Gomoob/Pushwoosh/Model/Notification/Notification.php.

See also https://codeclimate.com/changelog/5670481aac74390098001828 because the `Notification` class has no design problems.
