Streamline deploy notifications in Capistrano deploys

[jeffkreeftmeijer]

We prefer the APP_REVISION environment variable over the notify_of_deploy task, but since there's no proper way to set environment variables on the server in a Capistrano deploy, that's proving to be difficult to set up. We need a nicer way to do this, so we can actively recommend users to switch to using the APP_REVISION environment variable.

As reported in support (private Intercom link).

[tombruijn]

Add a config option: revision: abcdef1
Or add a revision file APP_REVISON/.appsignal_revision with one line that reads the last revision
We will keep the APP_REVISION env var

Also relevant for Elixir appsignal/appsignal-elixir#215

Discuss when @thijsc is back

[tombruijn]

We now have a revision config option: https://docs.appsignal.com/ruby/configuration/options.html#option-revision
We still should look if there's a way to set it from the capistrano config, which sets it on the deployed servers.