letsencrypt will prod on port 80 to verify the domain. `listen`
overrides `addSSL`, so none of the NixOS modules' setup will actually
work.
This means the conduit virtualhost never listened on port 80, and
couldn't verify letsencrypt requests.
How this *ever* worked is beyond me, but this commit resolves the
problems (don't worry, `forceSSL` does what it says on the tin and
overrides the `listen` again).
Tristan Daniël Maat
