Use ng-annotate instead of ngmin

ngmin is deprecated and ng-annotate is the recommended replacement (https://github.com/olov/ng-annotate).

From ng-annotate [README.md](https://github.com/olov/ng-annotate/blob/master/README.md):
"Grunt users can migrate easily by installing [grunt-ng-annotate](https://www.npmjs.org/package/grunt-ng-annotate) and replacing `ngmin` with `ngAnnotate` in their Gruntfile."