Fix to make filetime_from_git work with GitPython 0.3.2.1

When using the _filetime_from_git_ plugin with GitPython 0.3.2.1 installed, I encountered the following error message: 
```
 'Repo' object has no attribute 'commits'
```

This is just a quick fix to make it work with the most recent version of GitPython. When I run pelican now, it works as described, but warns about

```
Exception ImportError: ImportError('No module named sys',) in <bound method WindowCursor.__del__ of <smmap.mman.WindowCursor object at 0x10a2de368>> ignored
```

However, this is something that is either related to my environment or to smmap as a dependency. Nevertheless, I'd be happy to get any hints on how to fix it. :-)