gem: upgrade to Sprockets 4

we want the upgrade because it moves us away from the deprecated Ruby
`sass` (although still need to include it for compatibility
purposes[1]), gives us lots of speed and fixes a bug where a govuk
mixin cannot be resolved.

[1]: middleman/middleman-sprockets#132

govuk_tech_docs.gemspec

@@ -32,6 +32,8 @@ Gem::Specification.new do |spec|
   spec.add_dependency "middleman-compass", ">= 4.0.0"
   spec.add_dependency "middleman-livereload"
   spec.add_dependency "middleman-sprockets", "~> 4.0.0"
+  spec.add_dependency "sprockets", "4.0.0.beta10"
+  spec.add_dependency "sass"
   spec.add_dependency "middleman-syntax", "~> 3.0.0"
   spec.add_dependency "middleman-search-gds"
   spec.add_dependency "nokogiri"