Backport Rails 3.2 support

sprockets-rails 3.x is only compatible with Rails 4.x and greater.

It shouldn't be to hard to backport the 3.2 shims to get this working again.

/cc @charliesome 
Needs https://github.com/github/rails/commit/7f3ec3496d77751b227053c89ea1b5d87dcd7b04 to remove hard sprockets dependency.