Fix Cucumber tests to run migrations in test env

Under the Rails 4.1 appraisal the Cucumber tests are failing because of
this error:

    Migrations are pending. To resolve this issue, run:

        bin/rake db:migrate RAILS_ENV=test

features/rails_integration.feature

@@ -12,7 +12,7 @@ Feature: integrate with Rails
         end
       end
       """
-    When I successfully run `bundle exec rake db:migrate --trace`
+    When I successfully run `bundle exec rake db:migrate db:test:prepare --trace`
     And I write to "app/models/user.rb" with:
       """
       class User < ActiveRecord::Base