Fix invocation split once and for all

[kerrizor]

$ erd and $ rake erd are handled by 2 completely different codepaths. This is an oversight that is causing a lot of pain, both for our users and for me trying to troubleshoot people's problems. This needs to be fixed in the next point release.

[giovannibonetti]

In case it might be useful to anyone, in the meantime I'm using this workaround to run erd (and not rake erd) automatically after migrations:

# lib/tasks/db.rake
Rake::Task['db:migrate'].enhance do
  system('erd') if Rails.env.development?
end