Fix invalid parameter error on check for updates

See https://github.com/sparkle-project/Sparkle/issues/237#issuecomment-11618894

Got this error tonight with Sparkle 1.8.0 in a [project](https://github.com/editxt/editxt/tree/1.7.1) using Python 3.3/py2app 0.9. Got the error on both Mac OS X 10.8 and 10.9, although only intermittently on 10.8.

```
ObjC exception NSInternalInconsistencyException (reason: Invalid parameter not satisfying: windowNibPath) discarded
Stack trace (most recent call last):
  0x608000216a20
  ffi_call_unix64 (in _objc.so) + 79
  -[SUUpdater checkForUpdates:] (in Sparkle) (SUUpdater.m:324)
  -[SUUpdater checkForUpdatesWithDriver:] (in Sparkle) (SUUpdater.m:361)
  -[SUUserInitiatedUpdateDriver checkForUpdatesAtURL:host:] (in Sparkle) (SUUserInitiatedUpdateDriver.m:43)
  -[SUStatusController initWithHost:] (in Sparkle) (SUStatusController.m:34)
  -[SUWindowController initWithHost:windowNibName:] (in Sparkle) (SUWindowController.m:23)
  -[NSWindowController initWithWindowNibPath:owner:] (in AppKit) + 139
  -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] (in Foundation) + 189
  +[NSException raise:format:arguments:] (in CoreFoundation) + 104
  objc_exception_throw (in libobjc.A.dylib) + 43
  NSExceptionHandlerExceptionRaiser (in ExceptionHandling) + 172
```