Javascript does not get registered

Trying out the plugin the current grav version I had the problem that the javascript was missing after the first call.

Changing the code in MapShortcode.php to 

```
$this->shortcode->addAssets('js', '//maps.googleapis.com/maps/api/js'.$apikeystring);
$this->shortcode->addAssets('js', 'plugin://google-maps/js/google-maps.js');
```

helped.