Upgrade Sass gem dependency 3.2.7 -> 3.2.8

There are no major changes to the parse tree in this release, so it
is safe to upgrade.

Change-Id: I7cc03307baf76aba244c7ee5dd7ee679f2d62bbb
Reviewed-on: https://gerrit.causes.com/23187
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>