Added support for IPv6 addresses

This gem is currently not able to handle IPv6 addresses. We switched to the binary db, because the old DB doesn't support them. The fix is rather easy, just do the conversion to IPv6 only for IPv4 addresses.

I also reorganized the specs a bit (one expectation per spec) and added a Code Climate badge. If you want to, I could also setup a Travis CI job to automatically test all commits there.