Update MorphPivot.php

Missing Builder...

Got this error without:

```
ErrorException
Declaration of Illuminate\Database\Eloquent\Relations\MorphPivot::setKeysForSaveQuery() should be compatible with that of Illuminate\Database\Eloquent\Relations\Pivot::setKeysForSaveQuery()
open: /home/site/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Relations/MorphPivot.php
	public function setMorphType($morphType)
	{
		$this->morphType = $morphType;
 
		return $this;
	}
 
}
```