Enable source maps for stack traces in tests

Makes it easier to debug the more recent browserify modules, by enabling proper stack traces pointing to original source files (by using source maps), e.g.:

```bash
new_dashboard/header/settings_dropdown_view should have no leaks
TypeError: Cannot read property 'toLowerCase' of undefined
    at [object Object].module.exports.cdb.admin.DropdownMenu.extend.render (lib/assets/javascripts/cartodb/new_dashboard/header/settings_dropdown_view.js:38:1)
    at Object.<anonymous> (lib/assets/test/spec/cartodb/new_dashboard/header/settings_dropdown_view.spec.js:31:1)
    at attemptSync (http://0.0.0.0:8089/.grunt/grunt-contrib-jasmine/jasmine.js:1759:24)
    at QueueRunner.run (http://0.0.0.0:8089/.grunt/grunt-contrib-jasmine/jasmine.js:1747:9)
    at QueueRunner.execute (http://0.0.0.0:8089/.grunt/grunt-contrib-jasmine/jasmine.js:1733:10)
    at Spec.Env.queueRunnerFactory (http://0.0.0.0:8089/.grunt/grunt-contrib-jasmine/jasmine.js:569:35)
    at Spec.execute (http://0.0.0.0:8089/.grunt/grunt-contrib-jasmine/jasmine.js:318:10)
    at Object.fn (http://0.0.0.0:8089/.grunt/grunt-contrib-jasmine/jasmine.js:2072:43)
    at attemptAsync (http://0.0.0.0:8089/.grunt/grunt-contrib-jasmine/jasmine.js:1789:24)
    at QueueRunner.run (http://0.0.0.0:8089/.grunt/grunt-contrib-jasmine/jasmine.js:1745:16)
```