Investigate gitlab.com caching in CI

[peace-maker]

The proxy service in CI introduced in #2487 always shows cache misses for 404 responses of gitlab.com. I tried to have them cached too but adding the 404 http status code wasn't enough for gitlab.com apparently. Maybe some Cache control headers in the response we have to ignore?

pwntools/travis/libcdb_nginx_cache.conf

Line 64 in 3303ea9

proxy_cache_valid 200 404 12w;