fix chart not resetting after config.options is changed

I was getting problems like series not showing up while using the following pattern:

```
var resetCount = 0;
// this is a trick. Whenever $scope.config.options changes (using angular.equals)
// highcharts-ng does initChart. So we change $scope.config.options.resetCount
// whenever our data completely changes.

$scope.$watch('settings.country', function (newValue) {
   resetCount++;
   $scope.chartConfig = {
      options: {resetCount: resetCount, tooltip: {shared: false}},
      series: [{id: "revenue", name: "revenue": ...}, ...]
   };
}
```

i.e. reusing the same chart id but expecting a completely new chart to be drawn.

This fixes that!