Use `system` instead of `Bundler.clean_system` for dress_code

Bundler.clean_system doesnt have BUNDLE_PATH set correctly

Change-Id: Ib2760786e5dfbc7909f55ddd5f401a96fcf2a430
Reviewed-on: https://gerrit.instructure.com/71927
Reviewed-by: Tyler Pickett <tpickett@instructure.com>
Tested-by: Jenkins
QA-Review: August Thornton <august@instructure.com>
Product-Review: Brad Horrocks <bhorrocks@instructure.com>

lib/tasks/css.rake

@@ -2,7 +2,7 @@ namespace :css do
   desc "Generate styleguide"
   task :styleguide do
     puts "--> creating styleguide"
-    Bundler.clean_system('bin/dress_code config/styleguide.yml')
+    system('bin/dress_code config/styleguide.yml')
     raise "error running dress_code" unless $?.success?
   end
 end