installer: Hardcode 127.0.0.1 for IPv4

My home network has IPv6 (thanks Comcast!) and as a consequence in
some situations client programs may try IPv6 connections (like
ostree).

But ostree trivial-httpd only listens on IPv4 (with the old SoupServer
API).

Therefore, let's hardcode 127.0.0.1.

Followup to #33