Use correct reference to context through the importer

Fixes #366 in the correct way, hopefully.

I can't cite my sources well, since Sprockets is very difficult to follow... but suffice to say the global options hash sent to the Tilt SASS template engine always has a reference to the Sprockets importer, and the importer always has a reference to the context. Tested with Sprockets 2.2.2 with and without Rails.

lib/bootstrap-sass/compass_functions.rb

@@ -18,7 +18,7 @@ module Sass::Script::Functions
       options[:sprockets][:context]
     else
       # Compatibility with sprockets pre 2.10.0
-      options[:custom][:sprockets_context]
+      options[:importer].context
     end
   end
 end