I apparently had fog-1.18.0 installed when I saw this error. (Restarting an aws vagrant project I installed a while ago)
My naive attempt at a fix was to start with an upgrade
gem install fog
which upgraded something to fog-1.21.0. As it warned, "This could take a while..."
... and that did not help.
Exactly WHERE to add "gem 'unf'" wasn't clear from the solution voted up here, it seemed to be lib/fog/aws.rb but that was already there when I looked.
gem install unf
appeared to add it somewhere, but the problem did not go away.
I upgraded vagrant itself (1.4.3 to 1.5.1) and THAT didn't fix it.
Eventually, the fix was
vagrant plugin install unf
as I found in a thread at https://github.com/mitchellh/vagrant/issues/2507
I'm not sure if any of my previous fumbling attempts were also necessary, so I noted them here anyway.