A good question! This is one which even native speakers can't agree on. Most of the time we use a plural verb because this is what sounds most natural. However, some grammarians will insist that the verb should be in the singular, as 'none' means 'not one'. So, which one should you use? My advice would be to use the plural form in informal contexts, because that's what the majority of native speakers do. Just remember, if you find yourself doing a formal grammar test, that the singular form tends to be considered more 'correct'. Sorry not to be more helpful about this.