Use ng-animate with ui-sref-active

I wanted to use `ng-animate` with ui-sref-active so I made a relatively simple change to the directive.

After looking at ViewDirective's use of the `$animate` service, I see that some rather non-trivial things were done for backwards compatibility, etc. I don't feel qualified to copy over that logic, or refactor that logic into a service, so you probably won't be merging my code as-is.

Still, it's here as a feature request / proof-of-concept.

I was very satisfied with the results in my project!
wouldn't this require the ngAnimate module?