Ensure cache dirs are always present

[Simon Podlipsky]

From time to time after a fresh app deploy I encountered

CuyZ\Valinor\Cache\Exception\CacheDirectoryNotWritable
Provided directory /home/www/app/13925/var/cache/prod/Valinor/classes is not writable.

We should ensure those directories are always present on app cache warmup.

They are ultimately created but when there's higher app load, I presume some race condition occurs and this errors pops up before the dir is created.

[Romain Canon]

Thanks for the report! Should be fixed by #407 which will be part of the next release. 😉