luaposix: bump to version 31

Releases are no longer named 5.1.X, just the X portion remains now.

Fixes autoreconf issues with the automake bump, however since the patch
is no longer required that's not an issue now.

[Peter: now needs IPv6]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>