Update ngClass.js

I don't know if this was intended, but I don't think the code was comparing the indexes correctly and it was causing some weird behavior when setting classes on ng-class-even and ng-class-odd.

See the following plunkr for an example of the bug.  http://plnkr.co/edit/VAi6DB0QdWRutNGnVdSL?p=preview

Select the Red Radio button option to see that when switching classes on rows the classes are not always switched properly.