Make it run on Heroku

A bit of jiggery-pokery to get a staging environment running on Heroku: [mas-rad-staging.herokuapp.com](https://mas-rad-staging.herokuapp.com/)

__NOTE__: this requires the use of our [custom bowndler buildpack](https://github.com/moneyadviceservice/heroku-buildpack-ruby-bowndler); some background information on buildpacks can be [found here](https://devcenter.heroku.com/articles/buildpacks)