Brix 310 mpl ui part2

if you're using `.classed` this should probably be:

```
previous
    .classed('active', this.responseIndex_ > 1)
    .classed('disabled`, this.responseIndex_ <= 1);
```
or if you just want to append it to the class value above:

```
 var previous = flipperCntr.append('span')
    .attr('class', 'icon icon-caret-left ' + (this.responseIndex_ > 1) ? 'active' : 'disabled');
```