Avoid seg fault with recent nixpkgs

Resolves #16 (closed) .

I wouldn't say I understand the semantics of C++, but this seems to fix the seg fault. My very cursory understanding is that the previous code was wrong because resp would be deleted when it went out of scope, whereas now it doesn't as it lives on the heap instead. Though this might leak Response values.

What confuses me is why this behaviour changed at all...