Update grunt-contrib-sass to make it work with recent Sass versions

Update grunt-contrib-sass to make it work with recent Sass versions (Current version is 3.4.7). 

Otherwise, the following error gets through when running grunt:

```
Running "sass:dist" (sass) task
[D] Task source: /Users/motin/Dev/Projects/Gapminder/gs-dev/cms/yiiapps/internal-yii-frontend/bower_components-git-repos/sir-trevor-js/node_modules/grunt-contrib-sass/tasks/sass.js
Error: Error generating source map: couldn't determine public URL for the source stylesheet.
         No filename is available so there's nothing for the source map to link to.
        on line  of standard input
  Use --trace for backtrace.
```

More info: https://github.com/gruntjs/grunt-contrib-sass/issues/25

(Btw, there is no mention about whether or not to commit the results of `grunt sass` in the [contribution guidelines](https://github.com/madebymany/sir-trevor-js/blob/master/contributing.md), if you need them removed I'll remove the last commit below)