Bump minimum Sass version to 3.4.1

There is a bug in the Sass parser that was causing one of the tests
introduced for `BangFormat` to break. It is fixed in this version.

Change-Id: Ied232e8a69bb4f0971e02607f710fd2527efe746
Reviewed-on: http://gerrit.causes.com/44540
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Tested-by: jenkins <jenkins@brigade.com>