Docker provisioner uses incorrect gpg url and does not follow redirects

[Reen Lokum]

Bug has been resolved in the latest vagrant version(s)
https://github.com/mitchellh/vagrant/commit/69b7200c359642c4222748ea03398517e7e4499e

However they are not backported to 1.6.

❯ lsb_release -rd
Description: Ubuntu 15.04
Release: 15.04

❯ apt-cache policy vagrant
vagrant:
  Installed: 1.6.5+dfsg1-2
  Candidate: 1.6.5+dfsg1-2
  Version table:
 *** 1.6.5+dfsg1-2 0
        500 http://nl.archive.ubuntu.com/ubuntu/ vivid/universe amd64 Packages
        100 /var/lib/dpkg/status

Expected:
Vagrant VM with docker container installed.

Actual:
Vagrant VM did not install docker

❯ vagrant provision
==> dockerhost: Running provisioner: docker...
    dockerhost: Installing Docker (latest) onto machine...
The following SSH command responded with a non-zero exit status.
Vagrant assumes that this means the command failed!

curl http://get.docker.io/gpg | apt-key add -

Stdout from the command:

Stderr from the command:

stdin: is not a tty
  % Total % Received % Xferd Average Speed Time Time Time Current
                                 Dload Upload Total Spent Left Speed
  0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0
gpg: no valid OpenPGP data found.