Get turbo-sprockets-rails3 back

`turbo-sprockets-rails3` was removed upon upgrading to Rails 3.2.11 (c6ca6461aae0ca91fa30c) because of version incompatibility. But I found that it was using `turbo-sprockets-rails3` `v0.2.12` while the 0.3.x series is perfectly compatible with the newest Rails.