Doesn't work with Git 1.7.4

Hi, thanks for making this great tool.  My Mac was running Git 1.7.3, and when I tried to run `grunt gh-pages`, I got this error:

````
Cloning git@github.com:jeremyckahn/3d-cards.git into .grunt/grunt-gh-pages/gh-pages/src
Cleaning
Fetching origin
Checking out origin/gh-pages
Warning: usage: git ls-remote [--heads] [--tags]  [-u <exec> | --upload-pack <exec>]
                     [-q|--quiet] [<repository> [<refs>...]]
 Use --force to continue.

Aborted due to warnings.
````

I have since updated Git to version 1.8.5.1, and everything is working swimmingly.  It would be helpful if the documentation listed the minimum Git version number that is required.  Do you know what it is?