Default --device flag error

The --device flag causes the run task to fail when no device is detected. 'emulator' does not seem to be a valid argument.

```
Running "phonegap:run:ios:emulator" (phonegap:run) task
[phonegap] compiling iOS...
[phonegap] successfully compiled iOS app
[phonegap] installing app onto device
   [error] An error occurred while running the ios project. Targeting a device is not supported currently.
```

By default this should be empty, and the --device flag omitted from the command.