Do not force IPv6 addresses with LuaSocket >= 3

If socket.tcp6 was valorized (and this is the case on luasocket 3), the
tools.bind helper function effectively forced the use of IPv6 addresses.
The "ipv6-v6only" option diretly maps to IPV6_V6ONLY and it only affects
the communication of an INET6 socket:

https://tools.ietf.org/html/rfc3493#section-5.3

The socket.bind API in LuaSocket 3 already handles inet and inet6
addresses, so there is no need for tools.bind:

https://github.com/diegonehab/luasocket/blob/v3.0-rc1/src/socket.lua#L27
Merge pull request #39 from ntd/pr5

Do not force IPv6 addresses with LuaSocket >= 3