Keep assets:precompile compatible with Rails 3.2

ruby_rake_task was being defined locally with a version
copied from an older version of Rails. The signature didn't match
the one from Rails 3.2 causing the error below.

rake aborted!
wrong number of arguments calling `ruby_rake_task` (2 for 1)
pathto/jruby/1.9/gems/actionpack-3.2.17/lib/sprockets/assets.rake:66:in `(root)'