Fix exception at roscore startup if python has ipv6 disabled.

Admittedly a python bug, but the workaround is already present in other parts of this file.
Fix exception at roslaunch startup if python is build without ipv6 support.

A bit different than rosgraph fix because we do not have use_ipv6 here. See e.g. http://bugs.python.org/issue16208#msg178764