Eliminate the need for an on_load block

Addreses https://github.com/rails/rails/issues/9461

Given that sprockets-rails/lib/sprockets/railtie.rb contains:

  require 'sprockets/rails/helper'

And sprockets-rails/lib/sprockets/ruby/helper.rb starts out with:

  require 'action_view'

It follows that an on_load hander is not required.