Changed fox URL to a public URL

If the fox submodule is not already installed, a "make" command will raise the following error:
```
-- Initializing/Updating FoX XML submodule...
Cloning into 'src/xml/fox'...
Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
Clone of 'git@github.com:mit-crpg/fox' into submodule path 'src/xml/fox' failed
```
A quick Google search found the solution here: http://stackoverflow.com/questions/8197089/fatal-error-when-updating-submodule-using-git