feeds.py: Change last_modified to SQLAlchemy

Change the last_modified check to SQLAlchemy so it works with SQLite
too.

feeds.py

@@ -47,9 +47,10 @@ def write(self, outfile, encoding):
 
 
 def package_last_modified(request, *args, **kwargs):
-    cursor = connection.cursor()
-    cursor.execute("SELECT MAX(last_update) FROM packages")
-    return cursor.fetchone()[0]
+    try:
+        return Package.objects.latest('last_update').last_update
+    except ObjectDoesNotExist:
+        return
 
 
 class PackageFeed(Feed):