[BUGFIX] Warning when running bundle exec rake db:test:prepare

Rails 4.1 changes the way the DB schema is maintained for the unit tests.
This changeset switches from ActiveRecord::Migration.check_pending! to
ActiveRecord::Migration.maintain_test_schema!.

We also need to add rake db:setup to the Travis CI before_script section
as Travis does not automatically create the DB for Rails 4.1 projects.