Better cache purge

[brainsqueezer]

I would like to suggest using https://github.com/mtorromeo/ngx_cache_purge which is a fork of the original currently being used https://github.com/FRiCKLE/ngx_cache_purge as cache module. This version allows purging cache entries using wildcards using the same syntax as the paid module. This a very useful feature for this module. Also this version is the better maintained fork of the original.