Use HTTPS to download docker's gpg key.

Vagrant if failing while trying to add docker gpg.

```
  default: 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
100   184  100   184    0     0     71      0  0:00:02  0:00:02 --:--:-- 18400
gpg: no valid OpenPGP data found.
```

```
$ curl http://get.docker.io/gpg
<html>
<head><title>301 Moved Permanently</title></head>
<body bgcolor="white">
<center><h1>301 Moved Permanently</h1></center>
<hr><center>nginx/1.6.2</center>
</body>
</html>
```

Update: The reason for this error is because they are forcing SSL on the URL.
Fixed #4569