Re-enable UPnP by default in gitian builds

IIRC this was the case with 0.8.6, so let's keep this to avoid the risk
of losing connectable nodes with 0.9 release.

Also our miniupnpc library was recently updated and I've heard
reports that it works better than before now.
Merge pull request #3762

93c3e21 Re-enable UPnP by default in gitian builds (Wladimir J. van der Laan)