Fix ability to set active_class for magellan plugin

Custom class was not being set per #3869 

Broken at [commit](https://github.com/devinmcinnis/foundation/commit/48c7f05e169ac7fd714e8c143ebc20504f76c1cb#diff-c7a3357463136dd9249e989454730526L19)

Based on the [Foundation JS Configuration docs](http://foundation.zurb.com/docs/javascript.html#configuration), this will now work.

```
$(document).foundation({
  magellan : {
    active_class: 'custom'
  }
});
```