Rails engine fails Travis continuous integration with migration error

I have a Rails engine which fails Travis continuous integration with the following error:

Multiple migrations have the name CreatePaymillOnRailsPlans

When installed, the engine works fine and migration scripts are not duplicated:

$ ls -l db/migrate/
total 8
-rw-rw-r-- 1 lsoave lsoave 228 dic 20 19:01 20131111122533_create_paymill_on_rails_plans.rb
-rw-rw-r-- 1 lsoave lsoave 272 dic 20 19:01 20131111132308_create_paymill_on_rails_subscriptions.rb

$ cat db/migrate/*.rb
class CreatePaymillOnRailsPlans < ActiveRecord::Migration
  def change
    create_table :paymill_on_rails_plans do |t|

      t.string :name
      t.string :price
      t.string :paymill_id

      t.timestamps
    end
  end
end
class CreatePaymillOnRailsSubscriptions < ActiveRecord::Migration
  def change
    create_table :paymill_on_rails_subscriptions do |t|

      t.string  :name
      t.string  :email
      t.integer :plan_id
      t.string  :paymill_id

      t.timestamps
    end
  end
end

How can I get rid of it and still mainteining the db migration test?

Follow the .travis.yml file:

lsoave@basenode:~/Github/paymill_on_rails$ cat .travis.yml
language: ruby
bundler_args: --without development
rvm:
  - 1.9.2
  - 1.9.3
  - 2.0.0
  - 2.1.0
  - 2.1.1
  - 2.1.2
  - 2.1.3
  - 2.1.4
  - 2.1.5
env:
  - DB=sqlite
  - DB=mysql
  - DB=postgresql
script: 
  - RAILS_ENV=test bundle exec rake paymill_on_rails:db:migrate --trace
  - bundle exec rspec spec/
before_script:
  - mysql -e 'create database my_app_test'
  - psql -c 'create database my_app_test' -U postgres
bundler_args: --binstubs=./bundler_stubs
services: