support users in a different db connection

[deyikong]

https://github.com/jrean/laravel-user-verification/blob/69527fdd760cdd29853846e03504b567aac58c8a/src/UserVerification.php#L271

https://github.com/jrean/laravel-user-verification/blob/69527fdd760cdd29853846e03504b567aac58c8a/src/UserVerification.php#L342

The above two places make it almost impossible to use a different connection than the default(actually it is possible to get around it by changing DB facade temporarily, but it's not the right way), so i would suggest changing the second constructor parameter to $connection,

1. then at places where it needs schema, just do $connection->getSchemaBuilder(),
2. and replace DB with $connections in the above places.
3. And of course, you need to modify the service provider slightly as well. I would create a PR if you want it.