JS Src map: ajusted to the Revision 3 Proposal

The [Source Map Revision 3 Proposal](https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k) demonstrates the use of source maps like this:

     //# sourceMappingURL=<url>

and WebEssentials previously generated a link like that, that Google Chrome - at least - doesn't understand:

    /*
    //# sourceMappingURL=<url>
    */
 
This PR will fix #1586