Phoenix 最初の第一歩 Mix phx.serverでerrorの解決方法

# ```mix phx.server``` が急にerrorを出し始めた時の解決方法

## 対象者
2018.10.23近辺でPhoenixのチュートリアルにて学習開始し始めた初学者向け

### なぜ書くのか？
Phoenixの公式ドキュメントと、フレームワークの構成が変更中の差分が発生している時に起こり得る初学者が辛い時期の差分解決としてのメモ

## [Up and Running](https://hexdocs.pm/phoenix/up_and_running.html#conten)で学習し始めで事件は起きた！

```
Start your Phoenix app with:

    $ mix phx.server
```
上記のコマンドを実行するも、以下のエラーがでた時の解決方法を記載しておきます。

```
warning: found quoted keyword "test" but the quotes are not required. Note that keywords are always atoms, even when quoted, and quotes should only be used to introduce keywords with foreign characters in them
  mix.exs:57

warning: please add the following dependency to your mix.exs:

    {:plug_cowboy, "~> 1.0"}

This dependency is required by Plug.Adapters.Cowboy
which you may be using directly or indirectly.
Note you no longer need to depend on :cowboy directly.

  (plug) lib/plug/adapters/cowboy.ex:44: Plug.Adapters.Cowboy.warn_and_raise/0
  (plug) lib/plug/adapters/cowboy.ex:29: Plug.Adapters.Cowboy.child_spec/4
  (phoenix) lib/phoenix/endpoint/cowboy_handler.ex:81: Phoenix.Endpoint.CowboyHandler.child_spec/3
  (phoenix) lib/phoenix/endpoint/handler.ex:33: anonymous fn/5 in Phoenix.Endpoint.Handler.init/1
  (elixir) lib/enum.ex:1925: Enum."-reduce/3-lists^foldl/2-0-"/3
  (phoenix) lib/phoenix/endpoint/handler.ex:31: Phoenix.Endpoint.Handler.init/1
  (stdlib) supervisor.erl:295: :supervisor.init/1
  (stdlib) gen_server.erl:374: :gen_server.init_it/2
  (stdlib) gen_server.erl:342: :gen_server.init_it/6
  (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3

[info] Application hello_app exited: HelloApp.Application.start(:normal, []) returned an error: shutdown: failed to start child: HelloAppWeb.Endpoint
    ** (EXIT) shutdown: failed to start child: Phoenix.Endpoint.Handler
        ** (EXIT) an exception was raised:
            ** (RuntimeError) plug_cowboy dependency missing
                (plug) lib/plug/adapters/cowboy.ex:54: Plug.Adapters.Cowboy.warn_and_raise/0
                (plug) lib/plug/adapters/cowboy.ex:29: Plug.Adapters.Cowboy.child_spec/4
                (phoenix) lib/phoenix/endpoint/cowboy_handler.ex:81: Phoenix.Endpoint.CowboyHandler.child_spec/3
                (phoenix) lib/phoenix/endpoint/handler.ex:33: anonymous fn/5 in Phoenix.Endpoint.Handler.init/1
                (elixir) lib/enum.ex:1925: Enum."-reduce/3-lists^foldl/2-0-"/3
                (phoenix) lib/phoenix/endpoint/handler.ex:31: Phoenix.Endpoint.Handler.init/1
                (stdlib) supervisor.erl:295: :supervisor.init/1
                (stdlib) gen_server.erl:374: :gen_server.init_it/2
                (stdlib) gen_server.erl:342: :gen_server.init_it/6
                (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3
** (Mix) Could not start application hello_app: HelloApp.Application.start(:normal, []) returned an error: shutdown: failed to start child: HelloAppWeb.Endpoint
    ** (EXIT) shutdown: failed to start child: Phoenix.Endpoint.Handler
        ** (EXIT) an exception was raised:
            ** (RuntimeError) plug_cowboy dependency missing
                (plug) lib/plug/adapters/cowboy.ex:54: Plug.Adapters.Cowboy.warn_and_raise/0
                (plug) lib/plug/adapters/cowboy.ex:29: Plug.Adapters.Cowboy.child_spec/4
                (phoenix) lib/phoenix/endpoint/cowboy_handler.ex:81: Phoenix.Endpoint.CowboyHandler.child_spec/3
                (phoenix) lib/phoenix/endpoint/handler.ex:33: anonymous fn/5 in Phoenix.Endpoint.Handler.init/1
                (elixir) lib/enum.ex:1925: Enum."-reduce/3-lists^foldl/2-0-"/3
                (phoenix) lib/phoenix/endpoint/handler.ex:31: Phoenix.Endpoint.Handler.init/1
                (stdlib) supervisor.erl:295: :supervisor.init/1
                (stdlib) gen_server.erl:374: :gen_server.init_it/2
                (stdlib) gen_server.erl:342: :gen_server.init_it/6
                (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3
```

*  mix.exsファイルに```{:plug_cowboy, "~> 1.0"}```を追加しろと記載あるので追加する
* ちなみに、```mix.exs```は作成されたフォルダの１階層目にある

```
README.md	assets		deps		mix.exs		priv
_build		config		lib		mix.lock	test
```
* mix.exsファイル内の以下の場所に追加

### Befor
```
# Type `mix help deps` for examples and options.
  defp deps do
    [
      {:phoenix, "~> 1.3.4"},
      {:phoenix_pubsub, "~> 1.0"},
      {:phoenix_ecto, "~> 3.2"},
      {:postgrex, ">= 0.0.0"},
      {:phoenix_html, "~> 2.10"},
      {:phoenix_live_reload, "~> 1.0", only: :dev},
      {:gettext, "~> 0.11"},
      {:cowboy, "~> 1.0"}
    ]
  end
```

### After

```
# Type `mix help deps` for examples and options.
  defp deps do
    [
      {:phoenix, "~> 1.3.4"},
      {:phoenix_pubsub, "~> 1.0"},
      {:phoenix_ecto, "~> 3.2"},
      {:postgrex, ">= 0.0.0"},
      {:phoenix_html, "~> 2.10"},
      {:phoenix_live_reload, "~> 1.0", only: :dev},
      {:gettext, "~> 0.11"},
      {:cowboy, "~> 1.0"},
      {:plug_cowboy, "~> 1.0"}
    ]
  end
```
書き換えたら``` mix deps.get ```する。
色々と出るが、最後に以下が出てきたら成功

```
New:
  plug_cowboy 1.0.0
* Getting plug_cowboy (Hex package)
```
これでOK, ```mix phx.server ```しよう！

僕もPhoneixは勉強し始めた所なので、もし良かったら一緒に勉強しましょう。
毎週火曜日の午前中にオンラインで勉強会してます。また、月に１回集まってやってます。