Update to use new //# sourceMappingURL syntax

We were seeing an error in IE9 because grunt-concat-sourcemap was generating the //@ syntax for sourcemaps instead of the newer //# syntax.

This pull request changes it to //# instead.
This matches the syntax that grunt-contrib-uglify uses as well.

For more information see:
https://groups.google.com/forum/#!topic/mozilla.dev.js-sourcemap/4uo7Z5nTfUY
https://github.com/gruntjs/grunt-contrib-uglify/commit/0066be8645b97740bc74f58f904943198841ffd9