fix for windows masterless read-only highstate.cache.p file

After the first highstate run on windows masterless, all subsequent runs fail, because the highstate cache file is read-only with the error below:

IOError: [Errno 13] Permission denied: 'c:\\salt\\var\\cache\\salt\\minion\\highstate.cache.p'
Traceback (most recent call last):
  File "<string>", line 6, in <module>
  File "__main__.py", line 726, in <module>
  File "__main__.py", line 332, in bootstrap
  File "__main__.py", line 359, in chainload
  File "__main__.py", line 715, in _chainload
  File "__main__.py", line 128, in <module>
  File "__main__salt-call__.py", line 14, in <module>
  File "c:\salt\salt-0.17.4.win-amd64\salt-0.17.4-py2.7.egg\salt\scripts.py", line 82, in salt_call
    client.run()
  File "c:\salt\salt-0.17.4.win-amd64\salt-0.17.4-py2.7.egg\salt\cli\__init__.py", line 313, in run
    caller.run()
  File "c:\salt\salt-0.17.4.win-amd64\salt-0.17.4-py2.7.egg\salt\cli\caller.py", line 137, in run
    ret = self.call()
  File "c:\salt\salt-0.17.4.win-amd64\salt-0.17.4-py2.7.egg\salt\cli\caller.py", line 78, in call
    ret['return'] = func(*args, **kwargs)
  File "c:\salt\salt-0.17.4.win-amd64\salt-0.17.4-py2.7.egg\salt\modules\state.py", line 268, in highstate
    force=kwargs.get('force', False)
  File "c:\salt\salt-0.17.4.win-amd64\salt-0.17.4-py2.7.egg\salt\state.py", line 2468, in call_highstate
    with salt.utils.fopen(cfn, 'w+') as fp_:
  File "c:\salt\salt-0.17.4.win-amd64\salt-0.17.4-py2.7.egg\salt\utils\__init__.py", line 1030, in fopen
    fhandle = open(*args, **kwargs)
IOError: [Errno 13] Permission denied: 'c:\\salt\\var\\cache\\salt\\minion\\highstate.cache.p'