via: http://blog.pivotal.io/labs/labs/leave-your-migrations-in-your-rails-engines
in lib/[engine_name]/engine.rb
initializer :append_migrations do |app|
unless app.root.to_s.match root.to_s
config.paths["db/migrate"].expanded.each do |expanded_path|
app.config.paths["db/migrate"] << expanded_path