Use sprockets backport-compatible sass-rails fork

With 3.2.x sass-rails release, we hit NoMethodErrors with the Living
Style Guide attempting to handle asset helpers:

    undefined method `image_path' for nil:NilClass

This resolves the issue, for now, but moving to Rails 4 is the better
medium-term solution.

See also https://discussion.heroku.com/t/using-the-rails-4-asset-pipeline-in-rails-3-apps-for-faster-deploys/205

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Remove Sprockets::Helpers::RailsHelper usage

This Module is no longer present in sprockets-rails 2.0.0.backport1.
See previous commit c0dc50e9.

Signed-off-by: Alex Coles <alex@alexbcoles.com>