[5.0] Fix Incorrect DateTime Format

The problem is, The update of relation will refresh timestamp in two place,
The first one is in `Relations\HasOneOrMany::update`, Another is in
`Eloquent\Builder::addUpdatedAtColumn`.

Eloquent timestamp does not respect date format return by Model::getDateFormat()
if we update relation through model.

We can override getDateFormat() to return 'U' in model to
store datetime as unix timestamp. For example:

```php
use Illuminate\Database\Eloquent\Model;

class A extends Model
{
    public function b()
    {
        return $this->hasOne('B');
    }
}

class B extends Model
{
    public function getDateFormat()
    {
        return 'U';
    }
}

$a = A::find(9);
$a->b->update(array('foo' => 'bar')); // Invalid date format
```