Typehint for save() to follow Eloquent update

The following update to Eloquent\Model has changed the blueprint for the save method to have a typehint:
https://github.com/laravel/framework/commit/f6fdecf9af2059f064174b82afdef95bbc21d0fa#src/Illuminate/Database/Eloquent/Model.php

This needs to be reflected in the Ardent overriding save method.
Merge pull request #35 from duellsy/patch-2

Typehint for save() to follow Eloquent update