Apply undo list in reverse order.

In rare cases with hints with overlapping virtual packages provided by
different sources, this can make a difference.

Signed-off-by: Adam D. Barratt <adam@adam-barratt.org.uk>

britney.py

@@ -2326,6 +2326,7 @@ class Britney(object):
         else:
             self.output_write("FAILED\n")
             if not lundo: return
+            lundo.reverse()
 
             self.undo_changes(lundo, self.systems, self.sources, self.binaries)