Update Gruntfile.js

Source mapping was not working when I switched to SASS in C. Looks like they changed the property name.

Here's the article where i found the change. 
http://benfrain.com/lightning-fast-sass-compiling-with-libsass-node-sass-and-grunt-sass/
Merge pull request #17 from codenameEli/patch-1

Update Gruntfile.js