Updated wheezy (VM used by Vagrant) download URL

I am using vagrant for the first time, and when I tried to download the wheezy vm I ran into this issue:

```
MHDGAFDV13:designate_dev jord7642$ vagrant up
Bringing machine 'default' up with 'virtualbox' provider...
==> default: Box 'wheezy' could not be found. Attempting to find and install...
    default: Box Provider: virtualbox
    default: Box Version: >= 0
==> default: Adding box 'wheezy' (v0) for provider: virtualbox
    default: Downloading: https://www.dropbox.com/s/si19tbftilcuipz/debian-7.0-amd64.box
The box failed to unpackage properly. Please verify that the box
file you're trying to add is not corrupted and try again. The
output from attempting to unpackage (if any):

bsdtar: Error opening archive: Unrecognized archive format
```

After searching around, I found that someone fixed this issue by changing the download URL from "www.dropbox.com/..." to "dl.dropbox.com/...", and this fix worked for me.  I just wanted to have someone else check to make sure that they're not having any issues before this change is pulled.