when set_ticket is called the cached address to the handoff directory (handoff_dir) should be cleared (None)

Minor change:

code line broken at 80 characters for better readability and conformance with PEP8
handoff_dir is set to None when the ticket is set using set_ticket (was missed out in the previous commit)
Merge pull request #9 from hus787/4.1

when set_ticket is called the cached address to the handoff directory should ideally be cleared too