cp: non-portable warning

[csuhta]

As part of our Rails deploy on heroku-24, we now get this warning:

remote: -----> Detecting rake tasks
remote: -----> Preparing app for Rails asset pipeline
remote: cp: warning: behavior of -n is non-portable and may change in future; use --update=none instead
remote:        Running: rake assets:precompile

I am not sure if this is something we are doing or if it's something the buildpack is doing that cp on the heroku-24 stack is now warning about