Support airgapped CF deployments

[gberche-orange]

As a mongodb service operator, in order to run smoke tests in a CF deployment which does not have internet access to pull ruby gems dependencies, I need an offline release of the app

A travis-ci / circle-ci build which executes bundle package --all and uploads the resulting release to github

See more details about Vendor App Dependencies in https://docs.cloudfoundry.org/buildpacks/ruby/index.html#vendoring

See inspiration into https://github.com/orange-cloudfoundry/admin-ui-configurer/blob/master/.travis.yml