Closes #1

We can run the next Rails version in Heroku

➜  fastruby.io git:(debug/heroku-buildpack-ruby) heroku run bash --app fastruby-staging
Running bash on ⬢ fastruby-staging... up, run.7123 (Basic)
~ $ rails --version
Rails 7.1.0
~ $ echo $BUNDLE_GEMFILE
Gemfile.next

IMPORTANT NOTES:

• How are we going to manage this change in the long term? When the people at Heroku release new changes we are going to miss them out, is it better to keep our changes in a branch and rebase their changes as time passes? For sure, it would be better if we propose these changes to the official build pack and get merged 🤞

• Switching between the current and next Rails versions in the same Heroku instance is not straightforward, we should run a purge or clean command to remove previously used gems and not fall on an issue. But why do we need to switch between them? is that our main reason for supporting Gemfile.next? I don't think so.

• Even without these changes some tests were failing locally, it might be because of a misconfiguration, I wonder if we should make those tests pass and add more tests with the new implementation.

QA NOTES:

1. Select a project that already has the dual-boot configured and running.

2. In Heroku, go to the settings
   

Remove the official heroku/ruby build pack and include our build pack https://github.com/fastruby/heroku-buildpack-ruby#add_gemfile_next_support

Notice that right now we are using this branch to test the changes.

3. Again, in the settings go to the config vars section and add BUNDLE_GEMFILE as a key and Gemfile.next as a value, then click add.

Make sure Heroku picks the value you set in the config vars sometimes this takes some time.

4. In the overview section you should see that you added a new config key

5. Deploy the app, it should deploy the next version. since it would be hard to know what Rails version you are running with only seeing the web page, you can run heroku run bash --app your_app_name to get into the bash in Heroku.
   then run the rails --version, rails console, echo $BUNDLE_GEMFILE or any other command that helps you know you are using the next Rails version.