Fix implicit string conversion of Pathname

In Rails 3, ```Rails.public_path``` returns a String, but in Rails 4 it returns a Pathname. This breaks on the implicit conversion to String here: https://github.com/documentcloud/jammit/blob/master/lib/jammit/packager.rb#L113

If we explicitly instantiate DEFAULT_PUBLIC_ROOT as a String instead of a Pathname (as would happen in Rails 3 and earlier), the problem is solved.