Update SASS ( 3.3.5), Bourbon ( 4.0.2), Neat (1.6.0)

This work aims to update our Sass library dependency to 3.3.5. This significant Sass version requires an update of our other Sass dependencies as well (Bourbon and Neat).

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Note: this requires the following: 

* SASS 3.3+ 
* Bourbon 4.0.2
* Neat 1.6.0

so do ```bundle install```
That's not a Paver thing, it's a Python thing -- and yes, trailing commas are ignored.