correct routine to check container lives

Workaround is here:

1. use container has 5 seconds to launch at least. ( https://github.com/ainoya/flask-app/blob/master/app.py#L4 )
   Then replace repository url in Vagrantfile.

          config.vm.provision "shell" do |s|
             s.inline = $script
             # Set your repository for previewing by pool
             s.args = ["https://github.com/ainoya/flask-app.git"]
             # Set the maximum number of containers runnning at the same time
             s.args << "5"
           end
2. `vagrant ssh`
3. `open http://master.pool.dev` and confirm live-check works in you browser.