Made Jekyll serve IPv6 compatible by Default [mojombo:master => jekyll:master]

This is a fixed version of [this pull request](https://github.com/jekyll/jekyll/pull/1595#issuecomment-33178928) which was pointing at mojombo:master.

It contains a one line change to make Jekyll server IPv6 compatible by default.

@mattr- approved of it, but requested I repost the pull request.