Require doctrine/dbal for Laravel 4.1

Uhm… [doctrine/dbal](https://github.com/doctrine/dbal) seems to be [no longer included in Laravel 4.1](https://github.com/laravel/framework/blob/master/composer.json#L91). This causes an exception being thrown when using `artisan migrate`, because [Sentry uses `$table->renameColumn()` in the "alter throttle" migration](https://github.com/cartalyst/sentry/blob/develop/src/migrations/2013_11_26_025024_migration_cartalyst_sentry_alter_throttle.php#L33):

```php
$table->renameColumn('ip_address', 'ip');
```

Maybe let's require this library in the `composer.json`?