Stringify paths to unique paths correctly

The problem here is that gems like compass-sass and bootstrap-sass will
build their own Importer classes that aren't being properly uniqued.

Also this resolves the issue outlined in
sass#79

lib/sassc/rails/importer.rb

@@ -96,7 +96,9 @@ def imports(path, parent_path)
           return glob_imports(base, m[2], parent_path)
         end
 
-        search_paths = ([parent_dir] + load_paths).uniq
+        # Compass and other gems us their own special loaders
+        # Hence making this to_s required to have a proper uniq set.
+        search_paths = ([parent_dir] + load_paths).map(&:to_s).uniq
 
         if specified_dir != "."
           search_paths.map! do |path|