There seems to be an issue with vagrant 1.8.7 and the embedded curl version vs the mac os binary (shipped by default on mac os Sierra and others) – remove the embedded one
sudo rm /opt/vagrant/embedded/bin/curl
Note: you also need to remove the embedded curl
when you add a vagrant box (remotely or locally) so if you get the same error when running vagrant box add ....
just remove the curl from vagrant and it will work