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. It was lost in the autotools transition.

Also our miniupnpc library was recently updated (2f158df) from 1.6 to 1.8 and I've heard reports that it works better than before.