Fixing failed import of DjangoTestRunner 

Remove failing import of DjangoTestRunner from zen.py. The code works without it and it no longer exists in Django 1.5

This was the error i was seeing:
```
Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/home/tim/.virtualenvs/jammy/lib/python2.7/site-packages/django/core/management/__init__.py", line 453, in execute_from_command_line
    utility.execute()
  File "/home/tim/.virtualenvs/jammy/lib/python2.7/site-packages/django/core/management/__init__.py", line 392, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/tim/.virtualenvs/jammy/lib/python2.7/site-packages/django/core/management/__init__.py", line 272, in fetch_command
    klass = load_command_class(app_name, subcommand)
  File "/home/tim/.virtualenvs/jammy/lib/python2.7/site-packages/django/core/management/__init__.py", line 77, in load_command_class
    module = import_module('%s.management.commands.%s' % (app_name, name))
  File "/home/tim/.virtualenvs/jammy/lib/python2.7/site-packages/django/utils/importlib.py", line 35, in import_module
    __import__(name)
  File "/home/tim/.virtualenvs/jammy/lib/python2.7/site-packages/pyzen/management/commands/zen.py", line 6, in <module>
    from django.test.simple import DjangoTestRunner
ImportError: cannot import name DjangoTestRunner
```