v1.5.4+ no longer support read-only wp-cache-config.php

In https://github.com/Automattic/wp-super-cache/commit/3dd7420dc37ee37b27a029b5be16e6abe345b45e the possibility to have `wp-cache-config.php` read-only broke due to constant writes without checking permissions first.

This can be reproduced by removing write access to the `wp-cache-config.php` file and triggering GC by saving a post. It causes a white screen with the permission problem mentioned.

Inspect commit 3dd7420dc37ee37b27a029b5be16e6abe345b45e and the writes to wp-cache-config.php. Reproduce the problem by removing write access to that file, saving a post, and triggering GC. Done means the read-only configuration remains supported and saving the post no longer causes a permission-related white screen.

Written by the indexing model from the issue text.