www/nginx-module-cache-purge: Update to 3.0.3

Upstream has turned the snapshot into a release. It fixes a segfault on
PURGE requests in locations without a configured proxy cache, which now
return 404, keeps the separate purge location configuration inside "if"
blocks and adds regression coverage for inherited configuration and cache
key handling.

Changes: https://github.com/nginx-modules/ngx_cache_purge/releases/tag/3.0.3
Sponsored by: Netzkommune GmbH