Use environment variable for DB_SSLMODE for Postgres (laravel#6727)

```php
'pgsql' => [
   ...
    'sslmode' => env('DB_SSLMODE', 'prefer'),
]
```

Author: robsontenorio

config/database.php

@@ -95,7 +95,7 @@
             'prefix' => '',
             'prefix_indexes' => true,
             'search_path' => 'public',
-            'sslmode' => 'prefer',
+            'sslmode' => env('DB_SSLMODE', 'prefer'),
         ],
 
         'sqlsrv' => [