Update Theme URL to github

The Theme URL was previously pointing to an old Automattic svn repository that no longer exists. 
This change points the URL back to the main `_s` GitHub repository where it is now being developed on. 
Alternatively it could point to underscores.me
Update Theme URL

The Theme URL was previously pointing to an old Automattic svn repository that no longer exists. This change points the URL to underscores.me which is the site that promotes the theme.
Merge pull request #141 from jkudish/master

Update Theme URL to point to underscores.me. props @jkudish