IPv6 support for gateway.ipfs.io

[kyledrake]

Low-ish priority, but it shouldn't be too hard to setup.

I just enabled IPv6 on my router and was amazed to discover that my browser just auto-uses IPv6 by default without even telling me it was enabled. I had to install a plugin to even see if it was using 6, it's that transparent to end users now. Pretty cool.

[jbenet]

We should be able to add ipv6 support: https://www.digitalocean.com/community/tutorials/how-to-enable-ipv6-for-digitalocean-droplets -- at least in some of the droplets

[ghost]

duplicate of #88