don't expire cache dirs

There's not many of them, and there's a race condition where this script deletes
a dir at the same time the rails page cache wants to write a new file to it,
leading to 500s.

script/expire_page_cache

@@ -1,4 +1,3 @@
 #!/bin/sh
 
 find /srv/lobste.rs/http/tmp/cache/ -type f -not -mmin 2 -not -name ".gitkeep" -delete
-find /srv/lobste.rs/http/tmp/cache/ -type d -empty -delete