How can I make Ruby net::http stop stalling on ipv6?
I am not sure that this is an Apple issue, but a Ruby issue. You can see a bug report here. 

You might try specifying a ipv6-specific hostname upon initialization. 

For example, TCPServer.new('::1', pivotal_tracker_address) should allow you to use curl without specifying the -ipv6. I can't see you Ruby code, so I am not sure if this will fix your problem.

Hope this helps.