Welcome Message (contactRelationUpdate)

Original Issue: https://github.com/botman/botman/issues/970
-------------------
- BotMan Version: 2.4.1
- PHP Version: 7.3.1
- Messaging Service(s): BotFramework (Skype), Version 1.1.1
- Cache Driver: file

### Description:
Unable to trigger a Welcome Message using the the event fucntion ´on´ after adding my Bot in Skype. The propper event that should be fired is called `contactRelationUpdate`.

### Steps To Reproduce:
Add to ´routes/botman.php´
```
$botman->on('contactRelationUpdate', function($payload, $bot) {
$bot->reply('Thanks for adding me :)');
});
```
Add your Skype Bot in Skype.