Bundle Install could not fetch specs from https://rubygems.org/

Just in case none of the above satisfies the next intrepid explorer, I thought I'd drop here that after I spent 4 hours on this doing variants of the search that landed me here, I finally discovering that IPV6 was the culprit, after finding this specific thread on help.rubygems.org. Solution? this (Fedora, Linux):

sudo sysctl -w net.ipv6.conf.all.disable_ipv6=1
sudo sysctl -w net.ipv6.conf.default.disable_ipv6=1

set up a shell script to flip IPV6 on and off so I could run a command without it, and now everything runs peachy.

I had the same issue. The only working solution I found was to force http instead of https in Gemfile:

source 'http://rubygems.org'