Use latest sprockets-rails 3.2.x release

With the previous change we were only running CI builds against 3.0.x max.

Author: javierjulio

gemfiles/sprockets-rails_3_0.gemfile

@@ -1,6 +1,6 @@
 source 'https://rubygems.org'
 
-gem "sprockets-rails", "~> 3.0.0"
+gem "sprockets-rails", "~> 3.2"
 
 # Specify your gem's dependencies in sassc-rails.gemspec
 gemspec path: "../"