The method RedisSessionModule::destroy is supposed to return a bool

Without this change, the following php fails in session_destroy :
```
<?php
ini_set('session.save_handler', 'redis');
ini_set('session.save_path', 'tcp://127.0.0.1:6379?database=5&prefix=PHPREDIS_SESSION:');
session_start();
session_destroy();
?>
```
because $redis->del($key) returns a long and not a boolean