Update to RuboCop ~> 0.23
Bump to et_rails_app ~> 5.1.4
Add RuboCop config
Update to ChefSpec 4.0
Convert specs to RSpec 3.0.2 syntax with Transpec

This conversion is done by Transpec 2.2.4 with the following command:
    transpec

* 4 conversions
    from: obj.stub(:message)
      to: allow(obj).to receive(:message)

For more details: https://github.com/yujinakayama/transpec#supported-conversions
Add missing command stub to ChefSpec tests
Update Serverspec tests for Rspec 3.0.1