Avoid Arbitrary File Deletion abuse via Object Injection

[mcdruid]

As discussed with @rhukster this simple change should avoid \Grav\Framework\Cache\Adapter\FileCache::__destruct being abused via Object Injection to achieve Arbitrary File Deletion.

I've used strpos rather than str_starts_with as it looks like Grav supports PHP 7.

[rhukster]

@codex review