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>

Gemfile

@@ -107,7 +107,7 @@ group :production do
 end
 
 group :assets do
-  gem 'sass-rails',   '~> 3.2.3'
+  gem 'sass-rails', git: 'https://github.com/guilleiguaran/sass-rails.git', branch: 'backport'
   gem 'sass',         '~> 3.3.6'
   gem 'bourbon',      '~> 4.0'
   gem 'coffee-rails', '~> 3.2.1'

Gemfile.lock

@@ -20,6 +20,16 @@ GIT
     i18n-js (3.0.0.rc5)
       i18n
 
+GIT
+  remote: https://github.com/guilleiguaran/sass-rails.git
+  revision: 0393d9e1a6e4a776e61f16fb75f97971b9f4abd2
+  branch: backport
+  specs:
+    sass-rails (4.0.1)
+      railties (>= 3.2.0, < 5.0)
+      sass (>= 3.2.0)
+      sprockets-rails (~> 2.0.0.backport1)
+
 GIT
   remote: https://github.com/rails/prototype_legacy_helper.git
   revision: a2cd95c3e3c1a4f7a9566efdab5ce59c886cb05f
@@ -302,10 +312,6 @@ GEM
       structured_warnings (>= 0.1.3)
     rubyzip (1.1.0)
     sass (3.3.7)
-    sass-rails (3.2.6)
-      railties (~> 3.2.0)
-      sass (>= 3.1.10)
-      tilt (~> 1.3)
     selenium-webdriver (2.39.0)
       childprocess (>= 0.2.5)
       multi_json (~> 1.0)
@@ -328,6 +334,10 @@ GEM
       multi_json (~> 1.0)
       rack (~> 1.0)
       tilt (~> 1.1, != 1.3.0)
+    sprockets-rails (2.0.0.backport1)
+      actionpack (>= 3.0)
+      activesupport (>= 3.0)
+      sprockets (~> 2.2.2.backport2)
     sqlite3 (1.3.7)
     strong_parameters (0.2.1)
       actionpack (~> 3.0)
@@ -430,7 +440,7 @@ DEPENDENCIES
   ruby-progressbar
   rubytree (~> 0.8.3)
   sass (~> 3.3.6)
-  sass-rails (~> 3.2.3)
+  sass-rails!
   selenium-webdriver
   shoulda
   shoulda-matchers