Generate safe sourcemaps

The previous comment syntax was unsafe with Internet Explorer. This change
copies the equivalent upstream change to use the more robust nested comment:

```javascript
/*
//@ sourceMappingURL=file.min.js.map
*/
```

See:
*   https://github.com/mishoo/UglifyJS2/issues/108
*   http://bugs.jquery.com/ticket/13274

One aside: it seems like we should be able to move closer in line with the upstream UglifyJS2 but it appears that this is hard-coded into their bin script:

https://github.com/mishoo/UglifyJS2/blob/master/bin/uglifyjs#L301

Perhaps it's worth starting a dialog upstream to move that into the output stream where it belongs so we could simply configure it rather than copy-and-pasting?