[4.2] Allows Developer To Specify Connection To Use For Cache

Laravel allows developer to specify connection to be used by cache when using Database Driver. But, the same can be true for Redis Driver as well. In production, a good app will use a separate Redis server for cache from those used by some other things, say Queues (https://github.com/laravel/framework/pull/6035).

This pull request re-uses the `connection` option in `cache.php` config file (which was used by Database driver) to specify the connection for Redis driver as well.