[4.2] Let Carbon Do The Work For DateTime

Carbon have already a lot of test to retrieve the Datetime value, let him do the work
Nah, they failed:
```
DatabaseEloquentModelTest::testTimestampsAreReturnedAsObjectsOnCreate

Undefined variable: format

/home/travis/build/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:2577
/home/travis/build/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:2540
/home/travis/build/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:407
/home/travis/build/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:249
/home/travis/build/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:446
/home/travis/build/laravel/framework/tests/Database/DatabaseEloquentModelTest.php:298
```