workshops/views.py: Specify sort key explicitly in match()

Avoid:

  Traceback:
    File "/.../django/core/handlers/base.py" in get_response
      111. response = wrapped_callback(request, *callback_args, **callback_kwargs)
    File "/home/wking/src/swc/amy/workshops/views.py" in match
      262. persons.sort()
  Exception Type: TypeError at /workshops/match
  Exception Value: unorderable types: Person() < Person()

In Python 3.4.
Merge pull request #88 from wking/match-explicit-person-sort-key

workshops/views.py: Specify sort key explicitly in match()