[Sass] Fix manually enabling Sass::Plugin::Rack under Rails.

doc-src/SASS_CHANGELOG.md

@@ -5,6 +5,9 @@
 
 ## 3.0.1 (Unreleased)
 
+* Make sure manually importing the Sass Rack plugin still works with Rails,
+  even though it's not necessary now.
+
 * Allow identifiers to begin with multiple underscores.
 
 ## 3.0.0

lib/sass/plugin/rack.rb

@@ -1,5 +1,3 @@
-require 'sass/plugin'
-
 module Sass
   module Plugin
     # Rack middleware for compiling Sass code.
@@ -45,3 +43,5 @@ module Sass
     end
   end
 end
+
+require 'sass/plugin'