Allow getting Ruby version from other than `master`

The way we started defining the Ruby version the app needs in https://github.com/coopdevs/donalo/pull/61 only relies on `master`. This means that if we happen to provision the server to be used by an app's branch other than `master`, we might provision the wrong ruby version.

See https://github.com/coopdevs/donalo/pull/61#discussion_r392072579 for possible solutions.

Start by reading pull request #61 and the linked discussion for the proposed ways to select the Ruby version when provisioning a non-master branch. Trace the provisioning entry point described there and confirm that each app branch uses its own required Ruby version rather than master’s; validate the behavior across master and another branch.

Written by the indexing model from the issue text.