Update third-party cache_purge module from 1.4 to 1.5.
* Fix on-disk cache size calculation.
Since the initial release, recorded on-disk cache size was
decreased twice for purged content (once during cache purge
and once during subsequent cache update).
This resulted in recorded on-disk cache size being much
smaller than in reality, which could lead to on-disk cache
outgrowing defined "max_size" parameter.
Patch from Pyry Hakulinen.