Exclude .map output files from clean: grunt task

We're compiling Sass files on Windows with `grunt-sass`, not with `grunt-contrib-sass` as originally present in Gruntfile. So we're not 100% sure on what happens with original build file, but we noticed 2 things:

 1. ng-admin index page shows errors in console as it's looking for the map file
 1. the grunt file, while building, runs a clean task that removes all files in `build\` apart from minimised JS and CSS, thus deleting any MAP file possibly generated

That's why we thought it should be correct to preserve a MAP file during build.