If I have a mix.exs file something like:

defmodule Mix.Tasks.My_task do
  use Mix.Task

  @shortdoc "Perform my task"

  def run(_) do
    IO.puts "Working"
  end
end

defmodule ElixirKoans.Mixfile do
  use Mix.Project

  def project do

  ...    

end

I can happily run this with mix my_task.

How do I make my_task be the default, so it is executed when I run mix without a task?