fixed sqlite field nullability 

... so that it's usage will line up with the rest of the drivers.

The issue can be seen on this Laravel 3 pull request here:
https://github.com/laravel/laravel/pull/393

The problem is that if you don't explicitly declare nullable() in your migrations when using SQLITE, it'll fatally error if you do not assign a field. 

So, if you have 10 fields in your users table and try to do.. 

    User::create(['email' => 'bob@bob.com']);

It'll fail because of the other 9 fields.