Fix Travis CI

The [Travis CI](https://travis-ci.org/spree/spree_gateway) build is erroring for two reasons:

- Both [`money`](https://github.com/RubyMoney/money) and [`spree`](https://github.com/spree/spree) no longer support Ruby 1.8.x: [6503874](https://travis-ci.org/spree/spree_gateway/jobs/6503874)
- The `InstallGenerator` is not automatically running migrations even though it's [being asked to](https://github.com/spree/spree/blob/master/core/lib/spree/testing_support/common_rake.rb#L30) run them: [6503875](https://travis-ci.org/spree/spree_gateway/jobs/6503875)

The build will be red after this merge, which is better than an error. The [red build is unrelated](https://github.com/spree/spree_gateway/commit/66412a862d743ab3a4ef8225fef3dbc316285786) to Travis CI being broken.