allow falling-sky to run behind a reverse proxy using Apache >= 2.4

If one configures falling-sky to run behind e.g. nginx, then the
user's IP address will be falsely reported as 127.0.0.1 or ::1, even
if the X-Forwarded-For and X-Real-IP headers are set correctly.

This patch fixes the problem iff compiled against Apache >= 2.4.