It'd be better if the cache directory were located in wp-content/cache/wp-minify. It has become more standard that cache information be stored here. If not there at least use something other than a hardcoded path to your current cache location. Like
__DIR__ . '/cache/', a lot of people have their wp-content directory renamed, so your plugin really shouldn't have it hardcoded.
If you were going to move it, I'd recommend something like
WP_CONTENT_DIR . '/cache/wp-minify';