Investigate adding Bundler buildpack to configuration

[nwalberts]

Currently, when a developer deploys to Heroku, they will get an error:
You must use Bundler 2 or greater with this lockfile.

This can be solved by running heroku buildpacks:set https://github.com/bundler/heroku-buildpack-bundler2 in their project directory.

Investigation: there may be a way that we can set the correct buildpack for Bundler v2 from make_it_so (perhaps in the Procfile?). That way developers dont need to worry about that item for deployment to production.

Similar Issue on SO:
https://stackoverflow.com/questions/53759343/cant-git-push-heroku-master-after-i-tried-to-change-one-heroku-domain-to-another

Heroku Buildpack Bundler 2
https://github.com/bundler/heroku-buildpack-bundler2