Add DB::getConnectionDriverName() from DatabaseManager.

This change should allow the following:

```php
Schema::create('users', function ($table) {
    $table->increments('id');

    // some fields.

    if (DB::getConnectionDriverName() === 'mysql') {
        $table->engine = 'innodb';
    }
});
```

Signed-off-by: crynobone <crynobone@gmail.com>