a conflict against ng-src

If I put `ng-src` after `src` in an `<img>` tag, the `grunt-rev` task will just ignore the `src` path but never rev the image loading.gif.

```
<img src="loading.gif" ng-src="{% xxx %}"/>
```

However codes below make `grunt-dev` work well

```
<img ng-src="{% xxx %}" src="loading.gif"/>
```

.
