I was revising your deploy.rb and I don't know why it happened.
My guess (if the problem were in migrations tasks) is something wrong with your release_path variable , but it don't really make sense to me. I say it because original migrations tasks do some checks to choose a directory or another for deploying:
task :migrate, :roles => :db, :only => { :primary => true } do
rake = fetch(:rake, "rake")
rails_env = fetch(:rails_env, "production")
migrate_env = fetch(:migrate_env, "")
migrate_target = fetch(:migrate_target, :latest)
directory = case migrate_target.to_sym
when :current then current_path
when :latest then latest_release
else raise ArgumentError, "unknown migration target #{migrate_target.inspect}"
run "cd #{directory} && #{rake} RAILS_ENV=#{rails_env} #{migrate_env} db:migrate"