Another approach to spec'ing message expectations without caring about return values, as described in http://tatey.com/2013/08/24/dont-care-about-the-return-value-of-outgoing-commands/

restart_process_spec.rb

describe Task::RestartProcesses do
  describe '#run_with_error' do
    let(:client) { double('HerokuClient').as_null_object }
    let(:logger) { double('Logger').as_null_ojbect }
    let(:task)   { Task::RestartProcesses.new logger: logger }

    before do
      # Inject Dependencies
      task.client = client

      # Run
      task.run_with_error
    end

    it 'restarts processes' do
      expect(client).to have_received(:post_ps_restart)
    end

    it 'logs' do
      expect(logger).to have_received(:info).with('Restarting processes')
    end
  end
end